config: Replace '$GOOS' with 'GOOS', etc.
config: Replace '$GOOS' with 'GOOS', etc.

Windows' cmd.exe prefers wrapping in % (e.g. %GOOS%) [1], and Rob
Dolin has asked for the POSIX $ to be removed [2].  In this case, I
think the context is sufficient that we can just use the variable name
instead of Rob's recommended "for the environment variable FOO".

[1]: https://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/ntcmds_shelloverview.mspx?mfr=true
[2]: https://github.com/opencontainers/runtime-spec/issues/477#issue-157945714

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

laijs/specs,opencontainers/specs,hqhq/specs,opencontainers/runtime-spec,hqhq/runtime-spec,wking/opencontainer-runtime-spec,wking/opencontainer-specs