ci: Allow special branch names to prune CI jobs (#4604)

Quite often I find myself debugging code or CI specific to just one
platform. It makes for much longer iteration for a huge CI job that
reruns lots of tests that won't be affected by the changes I'm
experimenting with. Sometime I make a commit that places `if: 0` in
strategic places to exclude irrelevant batches of tests. But this is a
pain to add, and has to be removed again to re-enable the tests prior to
turning the experiment into a proper PR.

This PR, then, adds some logic to ci.yml that lets me do the same just
with special branch names, rather than needing to edit ci.yml. Any
branch you push containing the substring "windows-only" will not run any
Linux or MacOS CI tests. And the analogous behavior is added for
"macos-only" and "linux-only".

This shouldn't affect any CI jobs in which people are not purposely
using those special branch names. You should NOT use these special names
for branches from which you submit PRs, since obviously we want every PR
to validate on all CI combinations. But it is very helpful for topic
branches on your own fork as you are developing and iterating over many
CI runs but only need results from one platform.

Signed-off-by: Larry Gritz <lg@larrygritz.com>

.github/workflows/ci.yml

@@ -34,6 +34,7 @@ permissions: read-all
 jobs:
 
   aswf-old:
+    if: ${{ ! contains(github.ref, 'windows-only') && ! contains(github.ref, 'macos-only') }}
     name: "(old) ${{matrix.desc}}"
     strategy:
       fail-fast: false
@@ -241,6 +242,7 @@ jobs:
   # Linux Tests
   #
   linux:
+    if: ${{ ! contains(github.ref, 'windows-only') && ! contains(github.ref, 'macos-only') }}
     name: "${{matrix.desc}}"
     uses: ./.github/workflows/build-steps.yml
     with:
@@ -495,6 +497,7 @@ jobs:
   # MacOS Tests
   #
   macos:
+    if: ${{ ! contains(github.ref, 'windows-only') && ! contains(github.ref, 'linux-only') }}
     name: "${{matrix.desc}}"
     uses: ./.github/workflows/build-steps.yml
     with:
@@ -557,6 +560,7 @@ jobs:
   # Windows Tests
   #
   windows:
+    if: ${{ ! contains(github.ref, 'linux-only') && ! contains(github.ref, 'macos-only') }}
     name: "${{matrix.desc}}"
     uses: ./.github/workflows/build-steps.yml
     with: