Build process fix due to gftools no longer generating build.ninja

[iwsfutcmd]

Due to googlefonts/gftools@ebad929, gftools no longer produces a build.ninja file, meaning running ninja results in an error.

This PR fixes this by grabbing the unique .ninja filename from GFBuilder()

[iwsfutcmd]

note: fixes issue #39

[iwsfutcmd]

note: due to the change in gftools, the script generates a new build-#############.ninja file with every build, so if you build multiple times, your sources/ directory fills up with them. This wasn't an issue before because gftools just overwrote build.ninja with every build. I could add something to the script to delete the generated file after it's done, but wasn't sure if it would be necessary.

[iwsfutcmd]

ok, so with the fixes to the individual repos like these: notofonts/tangut@fd05a0c

this PR is no longer necessary as the build process doesn't seem to be using notobuilder.notobuilder at all. notobuilder.notoqa of course is still being used, but it means that I assume notobuilder.notobuilder could be removed entirely.

I'm going to leave this PR open for @simoncozens's thoughts on the matter, feel free to close whenever it's reasonable.

[simoncozens]

Yep, notobuilder.__main__ is dead now; building should just use gftools-builder. The module is sticking around for rebuilding the websites, making UI VFs etc, but this PR is now unnecessary. But thank you anyway!