Add workflow to create single source fpm version

[awvwgk]

This adds the script to generate the single source file version of fpm for bootstrapping. The following steps are used to generate the bootstrap version:

1. remove test targets from package manifest by patch (remove after Only compile tests for fpm test command #92 is resolved)
2. install an existing binary of fpm
3. call fpm with compiler wrapper script to collect single source version
4. build bootstrap version from single source version
5. reverse patch the package manifest (remove after Only compile tests for fpm test command #92 is resolved)
6. build fpm using the bootstrap version
7. upload single source file version

Finally, the single source file version is uploaded on releases.

This patch also changes the release workflow to run on every commit, but only upload on releases. This allows to use the created artifacts from PRs to quickly bootstrap a new fpm version with some new features implemented.

Requires #560
Closes #391
Closes #559

[awvwgk]

You can find generated single source file version here: https://github.com/fortran-lang/fpm/suites/3704688086/artifacts/90445473

Note that links to uploaded artifacts from workflows expire quite quickly, go to the Checks tab to select the correct artifact if the link above is broken.

[certik]

This is really good. Thank you @awvwgk for this.

With this in, I feel the bootstrap is very robust: the master branch will always bootstrap using itself and we have scripts to create the single file. Thus when packaging, we can even use the same scripts to create the single source file; or we can continue using an older version --- it does not matter at this point, because if anything breaks with an older version we know that we can always create the single source and it will work.

This is fine with me to merge as is.

[awvwgk]

Thanks for reviewing. I'll go ahead and merge now.