build.yml: Adjust for change in 'brew'

'brew' apparently introduced an incompatible change, where even
"--force" does not actually make the commands available in the default path.

Also switch to a numbered macos release instead of "latest", though this
did not save us from breaking changes in brew or other preinstalled sw.

.github/workflows/build.yml

@@ -94,15 +94,15 @@ jobs:
         path: mpy-cross/mpy-cross.static.exe
 
   mpy-cross-mac:
-    runs-on: macos-latest
+    runs-on: macos-10.15
     steps:
     - name: Dump GitHub context
       env:
         GITHUB_CONTEXT: ${{ toJson(github) }}
       run: echo "$GITHUB_CONTEXT"
-    - name: Install deps
+    - name: Make gettext programs available
       run: |
-        brew link --force gettext
+        echo "::set-env name=PATH::/usr/local/opt/gettext/bin:$PATH"
     - name: Versions
       run: |
         gcc --version