Mono build on non-windows cannot find xbuild if it is not in /usr/bin

[TheCakeIsNaOH]

Uppercut appears to hardcode the xbuild path to /usr/bin/xbuild:
https://github.com/chocolatey/choco/blob/master/.build/compile.step#L118

This is fine most of the time, however, if xbuild is not available there, the mono build will fail.
For example, the Github actions MacOS environment has xbuild in /Library/Frameworks/Mono.framework/Versions/Current/Commands

Removing the basedir appears to work, as then it just uses whatever xbuild is on the path, if I understand what is going on correctly.

Log of failing build:

     [echo] Removing and adding /Users/runner/work/choco/choco/build_output.
     [echo] Build for chocolatey started at 04/16/2021 20:13:58 with UppercuT.
     [echo] Using UppercuT version: 1.4.2.0.
     [echo] ====================
     [echo] UppercuTBuild
     [echo] ====================
                 [echo] ====================
                 [echo] GitVersioner
                 [echo] ====================
                 [echo] Getting revision number from working directory '/Users/runner/work/choco/choco' or remotely at https://github.com/chocolatey/choco.
                 [exec] fatal: No names found, cannot describe anything.
                 [exec] /Users/runner/work/choco/choco/.build/versioners/git.step(26,6):
                 [exec] External Program Failed: git (return code was 128)
     [echo] Using revision number: fatal: No names found, cannot describe anything. and hash: fatal: No names found, cannot describe anything..
     [echo] Using build number 0.
     [echo] Version full updated to 0.10.16.0.
     [echo] Compiled code drop directory will be /Users/runner/work/choco/choco/code_drop.
                 [echo] ====================
                 [echo] VersionBuilder
                 [echo] ====================
                 [echo] Generating a strong name key (/Users/runner/work/choco/choco/chocolatey.snk) using /Applications/Microsoft SDKs/Windows/v6.0A/Bin/sn.exe if the key file doesn't already exist.
                 [echo] Generating SolutionVersion.cs with assembly version 0.10.16.0.
                 [echo] ====================
                 [echo] BuildInformation
                 [echo] ====================
                 [echo] Generating XML file with some information to possibly get merged in at /Users/runner/work/choco/choco/build_output/_BuildInfo.xml.
                 [echo] ====================
                 [echo] Compiler
                 [echo] ====================
                        [echo] 
                        [echo] ====================
                        [echo] compile PRE Extension
                        [echo] ====================
                        [echo] Running '/Users/runner/work/choco/choco/.build.custom/compile.pre.step' - details will be in the build.log.
                             [echo] Copying NuGet.Core prior to compiling. This helps resolve issues with Mono 3.12.0.
                 [echo] Setting compile variables
                 [echo] Finding and restoring all packages (/Users/runner/work/choco/choco/lib/NuGet/NuGet.exe restore '/Users/runner/work/choco/choco/src/chocolatey.sln')
                 [echo] Compiling /Users/runner/work/choco/choco/src/chocolatey.sln.
                 [echo] Building on 4.5
                 [echo] Building...
                 [echo] Working directory /Users/runner/work/choco/choco/build_output
                 [echo] Running xbuild /Users/runner/work/choco/choco/src/chocolatey.sln /nologo /property:OutputPath='/Users/runner/work/choco/choco/build_output/chocolatey' /property:Configuration=Release /verbosity:detailed /toolsversion:4.5 /property:Platform='Any CPU' /property:TargetFrameworkVersion=v4.5 /l:ThoughtWorks.CruiseControl.MSBuild.XmlLogger,"/Users/runner/work/choco/choco/lib/NAnt/ThoughtWorks.CruiseControl.MSBuild.dll";'/Users/runner/work/choco/choco/build_output/build_artifacts/compile/msbuild-mono-4.5-results.xml'
            
            
            /Users/runner/work/choco/choco/.build/compile.step(117,8):
            '/usr/bin/xbuild' failed to start.
                ApplicationName='/usr/bin/xbuild', CommandLine='/Users/runner/work/choco/choco/src/chocolatey.sln /nologo /property:OutputPath='/Users/runner/work/choco/choco/build_output/chocolatey' /property:Configuration=Release /verbosity:detailed /toolsversion:4.0 /property:Platform='Any CPU'  /property:TargetFrameworkProfile='' /property:TargetFrameworkVersion=v4.5 /l:ThoughtWorks.CruiseControl.MSBuild.XmlLogger,"/Users/runner/work/choco/choco/lib/NAnt/ThoughtWorks.CruiseControl.MSBuild.dll";'/Users/runner/work/choco/choco/build_output/build_artifacts/compile/msbuild-mono-4.5-results.xml'', CurrentDirectory='/Users/runner/work/choco/choco/build_output', Native error= Cannot find the specified file