b2d865f180 introduced a call to exportVariable() to export the MSYS env
var, which configures the symlink strategy for MSYS2/cygwin binaries it calls.
By setting the env var globally, this also changes the behaviour of other MSYS2
using tools in a CI job, and also overrides MSYS configuration set by the user,
which I think was not intended.
To avoid this leakage set the MSYS env var only for the commands which
@actions/cache calls.
Fixes#1312
ea21da6993