Derp, use the example below

Author: saethlin

.github/workflows/ci.yml

@@ -34,11 +34,13 @@ jobs:
     steps:
       - uses: actions/checkout@v3
 
-      - name: Configure
-        run: echo "MIRIFLAGS=-Zmiri-tag-gc=$([[ ${{ matrix.os }} == \"ubuntu.latest\" ]]; echo $?)" >> $GITHUB_ENV
+      - name: Set the tag GC interval to 0 on linux
+        if: runner.os == 'linux'
+        run: echo "MIRIFLAGS=-Zmiri-tag-gc=1" >> $GITHUB_PATH
 
-      - name: Test
-        run: echo $MIRIFLAGS
+      - name: Disable the tag GC on non-linux
+        if: runner.os != 'linux'
+        run: echo "MIRIFLAGS=-Zmiri-tag-gc=0" >> $GITHUB_PATH
 
       # We install gnu-tar because BSD tar is buggy on macOS builders of GHA.
       # See <https://github.com/actions/cache/issues/403>.