Follow-up to SERVER-111404 (#63928). `buildscripts/tests/test_gdb_rbe_sysroot.py` fails `buildscripts_test` on Windows: it writes temp files with `<`/`>` in their names (from the byte-order format characters) and executes shell-script stubs, neither of which works on Windows.

`buildscripts/gdb_rbe_sysroot.py` only runs on Linux spawn hosts, so the test classes (`ElfParsingTest`, `CoreUsesRbeSysrootTest`, `BuildGdbCommandTest`) are now decorated with `unittest.skipUnless(sys.platform.startswith("linux"))`. Linux coverage is unchanged.