ci: spec22-gems msys harness roots the toolchain bin at the msys mount

[ronaldtse]

Why

The third arm64 dogfood (tebako-runtime-ruby run 35690023046) reached the gem-install leg with #112 + #113 in place and died there:

PROBE-PIPE path=…;D:\a\_temp\msys64\clangarm64\bin;C:\a\_temp\msys64\usr\bin;C:\Windows\System32
PROBE-PIPE host-tool make resolved=C:\a\_temp\msys64\usr\bin/make.exe
make: clang++: No such file or directory

The UCRT64_BIN default spelled the setup-msys2 install root as /d/a/_temp/msys64 — correct on the x64 runners (workspace on D:), wrong on windows-11-arm (workspace and toolcache on C:). The PATH carried a nonexistent D:\…\clangarm64\bin, so the runtime's recorded clang++ could not resolve and sassc's native ext failed with make error 127.

What

The default is now /<toolchain>/bin — POSIX spelling off the msys2 install-root mount. / is the install root in the msys2 shell, so the same spelling resolves on any runner drive layout, and it still rides the conv_envvar conversion into native spawns exactly as before. The variable keeps its name and its override contract; only the default changes.

Proof

• Failing run's own log shows the mismatch: PATH with the nonexistent D: entry vs make resolving under C:\a\_temp\msys64.
• bash -n clean; no other hardcoded install-root spellings remain (the one left is in the comment documenting this incident).
• x64 behavior unchanged: /ucrt64/bin resolves to the same directory the old default spelled out on D:-drive runners.