[New msys2-runtime version] 2 new items

[github-actions]

[New msys2-runtime version] CI: add missing libzstd-devel

CI: add missing libzstd-devel

See 1387ea9f984d5a7aa096a6 - our binutils now pulls in zstd, and
cygwin will only consider linking to it if it is present.
Install it so it is found at configure time.

msys2/msys2-runtime@f2d12f5

[New msys2-runtime version] cygcheck: remove an unused variable causing a build error with GCC 16

cygcheck: remove an unused variable causing a build error with GCC 16

Since gcc 16 this triggers -Werror=unused-but-set-variable=
Remove the unused variable.

msys2/msys2-runtime@fc2528f