[vcpkg-acquire-msys] Allow TOOLPATH to be detected/configured if MSYS2 installation is already present in the environment

[julianxhokaxhiu]

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Is your feature request related to a problem? Please describe.
At the current status, using vcpkg in CI environments is an issue as vcpkg expects to download an entire MSYS2 installation and configure it on each run. Not only this is a waste of resources ( CPU, Disk, Bandwidth ) but it wastes time as well as jobs will take longer to be completed.

AppVeyor, as CI provider, solved this by providing an already pre-configured MSYS2 environment in their agents, although it is currently not detected by vcpkg.

Proposed solution
The overall idea is allow vcpkg to configure the TOOLPATH externally, or ideally autodetect existing installations.

Describe alternatives you've considered
At the current state I'm simply overriding the vcpkg_acquire_msys.cmake file with a custom patched one which changes only this line: https://github.com/julianxhokaxhiu/ffmpegCI/blob/master/patch/scripts/cmake/vcpkg_acquire_msys.cmake#L39

Additional context

• AppVeyor build environments: https://www.appveyor.com/docs/windows-images-software/
• See also [vcpkg-acquire-msys] Improvement #11810

[ASxa86]

I believe this is similar if not the same thing I've been dealing with and had an issue made #7617. Major difference is, I don't have access to the internet.

[JackBoosY]

msys version will also be considered.
@emptyVoid What do you think?

[emptyVoid]

We could add an environment variable VCPKG_MSYS2_ROOT (or VCPKG_MSYS2_PATH) overridable in a triplet, which if present would leverage user-supplied MSYS2 installation.

I haven't found any means to get MSYS2 version, and according to this stackoverflow answer we could only operate on pacman version or version of some other core package (e.g. msys2-runtime). I would take 5.0.2 as a minimally viable pacman version throwing an error if user provided MSYS2 is older (as the new core system upgrade scheme is supported since 5.0.1.6403). And 5.2 as the required pacman version making a core system upgrade if needed.