--no-install-ghc also blocks install MSYS2 when necessary

[andreasabel]

Seems like --no-install-ghc stops stack also from installing MSYS2.

Observed in a CI run: https://github.com/haskell-hvr/brotli/actions/runs/12653647868/job/35259576693?pr=13#step:4:11

stack exec --stack-yaml=stack-9.10.yaml --no-terminal --no-install-ghc --system-ghc -- pacman --noconfirm -Sy msys2-keyring
Warning: Stack is not using a Stack-supplied MSYS2.
Executable named pacman not found on path: ...

The docs instead say that it only affects the instalation [sic!] of GHC:

stack/doc/configure/global_flags.md

Lines 143 to 149 in 9f1ba4e

	## `--[no-]install-ghc` flag
	Default: Enabled
	Enables/disables the download and instalation of GHC if necessary. For further
	information, see the documentation for the corresponding non-project specific
	configuration [option](yaml/non-project.md#install-ghc).

I suppose at least the documentation should be fixed, maybe the option should be differentiated.

[mpilgrem]

@andreasabel, I have fixed the documentation.

The current logic is that if skip-msys is true (false is the default), Stack-supplied MSYS2 is not used. Otherwise, if MSYS2 is missing and install-ghc is true (the default), Stack seeks to fetch MSYS2.

I agree with your suggestion. I think that means there should be:

• an install-msys with a default value equal to the (default or specified) value of install-ghc; and
• (as now) if skip-msys is true, Stack-supplied MSYS2 is not used;
• otherwise, if MSYS2 is missing and install-msys is true, Stack seeks to fetch MSYS2.

That way:

• the default experience would be unchanged, including when install-ghc is false (install-msys is also false); but

• a user could specify install-ghc: false and install-msys: true (overriding the default) to use their own GHC but still have Stack download and install the Stack-supplied MSYS2 when necessary.

[mpilgrem]

I've raised a pull request that seeks to implement this:

• Fix #6675 Add install-msys and ancillary matters #6677

[andreasabel]

Excellent, thanks!