Clean Cabal cache before calling it

Cabal/GHC doesn't know about files produced by cargo, and will therefore
fail to invalidate caches. While there are ways to tell Cabal/GHC to
depend on these files, they are known to be broken in our tool versions.

See: haskell/cabal#4746

Note that we don't use `cabal clean`, as it will also remove any sources
it downloaded (specified in `cabal.project`).

bittide-shake/bin/Shake.hs

@@ -334,6 +334,15 @@ main = do
               -- build bittide-instance because we have instances that includes a binaries.
               command_ [Cwd "firmware-binaries"] "cargo" ["build", "--release"]
 
+              -- XXX: Cabal/GHC doesn't know about files produced by cargo, and
+              --      will therefore fail to invalidate caches. While there are
+              --      ways to tell Cabal/GHC to depend on these files, they are
+              --      known to be broken in our tool versions.
+              --
+              --      See: https://github.com/haskell/cabal/issues/4746
+              command_ [] "rm" ["-rf", "dist-newstyle/build"]
+
+              -- Generate RTL
               let
                 (buildTool, buildToolArgs) =
                   defaultClashCmd clashBuildDir targetName