Find sh on the PATH when running under Git Bash or MSYS2 on Windows - #2289

When MSYSTEM is set (Git Bash, MSYS2), GetShellRunArray returns /bin/sh. mlr.exe is a native Windows program and can't resolve that MSYS path, so everything that shells out fails from Git Bash:

$ mlr -n put 'end { print system("echo hi") }'
(error)
$ mlr --icsv --ojson --prepipe cat cat example.csv
mlr: fork/exec /bin/sh: The system cannot find the path specified.

Piped redirects (tee | "cat", $*) go through the same function. This looks sh up on the PATH instead (Git Bash and MSYS2 put their usr/bin there) and falls back to cmd /c if it isn't found. exec.LookPath returns an absolute path, which os.StartProcess in halfpipe.go needs.

Added a Windows-only test in pkg/platform that sets MSYSTEM and runs echo hello through the result; it fails before the change and passes after (skipped when there's no sh on the PATH). TestRunAllowShell in pkg/terminals/mcp also failed from Git Bash before this.

mlr regtest from Git Bash still needs the mlr path with forward slashes in the sh -c line; I left that out to keep this small.