patch: .github/workflows

[bradfitz]

Summary: add the .github/workflows directory for Tailscale
Upstream status: not applicable. Forever forked.
Example prior commit: 4865b81

When cherry-picking that commit, be sure to add additional tailscale.go1.NN branches to build.yml as needed.

[bradfitz]

Actually, when cherry-picking this, don't. Instead just tar up the old directory and re-add it to the new tree. Otherwise don't forget any other edits to it, e.g. 954407f.

[bradfitz]

Example updating ts1.24 from the old go1.24 gitbub files:

$ git restore --source=tailscale.go1.23 .github/workflows .github/dependabot.yml
$ git add .github/workflows .github/dependabot.yml