This simplifies parsing when using these rules from scripts.

This is a backport of core#21304
https://github.com/bitcoin/bitcoin/pull/21304/commits/84912d4b24382ae022da3a863bd6caa2b8948d94

I was wondering why I had to add this space to find the SDK, when I initially tested guix.

With the Apple SDK in the expected directory:

HOSTS=x86_64-apple-darwin contrib/guix/guix-build

Check that it does not fail early because of not finding the SDK.