ci: stop cancelling master runs, and revert the release job conditions

The empty homebrew tap was never a condition problem. The release job ran, the
matrix built all six targets, and the tap step then failed with "Input required
and not supplied: token" because HOMEBREW_TAP_TOKEN resolves to an empty
string. Four rounds of edits to those `if` expressions chased a symptom, so
they go back to what they were.

What stays is the CI concurrency change: a cancelled master run leaves the
release gate waiting on a CI run for a commit nothing will push again.

.github/workflows/ci.yml

@@ -9,10 +9,6 @@ on:
   pull_request:
     branches: ['**']
 
-# Superseding a branch run saves the matrix from running twice, but a master run
-# must be allowed to finish: the release workflow gates on finding a completed
-# CI run for its own commit, and nothing ever pushes that commit again. A
-# cancelled run there strands the release until it times out.
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}
   cancel-in-progress: ${{ github.ref != 'refs/heads/master' }}