Create self contained source archives

[felfert]

Hi,

This resolves #50 by creating self contained source archives and preserving the tag in those in a file named .git-tag in order to provide reproducible builds. The relevant steps are also invoked in the linux github action.

Notes:

• This PR contains 2 commits intentionally. The first commit just removes the master branch limitation for the linux github action. This was necessary to prove functionality on my branch in my fork. You can either revert this commit after merge or notify me to revert it before you merge.
• The source archives are generated in srcdist/ (just like binaries in builds/)
• Since the automatic src archives generated by github can not be disabled (or can they?), you might want to put a note about choosing the correct source archives.

Cheers -Fritz

[todbot]

On initial inspection, looks really good. Much simpler than the path I was going down. Thank you. I'll be testing it across all the non-CI platforms soon.

And thanks for teaching me you can define Make functions in a single line. That's handy.