ci: clean up workflow shellcheck nits

ci: clean up workflow shellcheck nits

actionlint (rhysd/actionlint) reported a handful of shellcheck-class issues
across the GitHub Actions workflows.  All are 1-line mechanical fixes:

  * Replace legacy backticks in --rsync-bin=`pwd`/rsync with
    --rsync-bin="$PWD/rsync" (SC2006 + SC2046; almalinux-8-build,
    macos-build, ubuntu-22.04-build, ubuntu-build).
  * Quote >>$GITHUB_PATH redirects as >>"$GITHUB_PATH"
    (SC2086; coverage, macos-build, ubuntu-22.04-build, ubuntu-build).

After this commit `actionlint .github/workflows/*.yml` exits 0.

(Also cleaned up 6 editor backup *.yml~ files from the local working
tree; those weren't tracked -- *~ is gitignored -- so the cleanup is
local-only and not part of this commit.)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>