don't try-install on MacOS (#384)

Problem is that these runners are always slow, whereas the ones on
windows are only _sometimes_ slow. Generally we don't have to assume
installation fails on unix as long as we can build tests, so
installing on windows alone should be all that's needed to get
relevant coverage.

Author: Byron

.github/workflows/ci.yml

@@ -57,6 +57,7 @@ jobs:
         run: vcpkg install  zlib:x64-windows-static-md
         if: startsWith(matrix.os, 'windows')
       - name: "Installation from crates.io"
+        if: startsWith(matrix.os, 'windows')
         uses: actions-rs/cargo@v1
         with:
           command: install