Extract github actions for interesting rust toolchain versions

Extract a github action to reduce the number of references to our MSRV and
stable (to be pinned in the next commit).

While at it, use the MSRV for macOS builds; this means that we'll be less
like accidentally to break the macOS build when bumping the MSRV.  I don't
think there is a reason for using 1.73 specifically, other than "it's the
highest we can use on old macOS", so using an even older one should be fine.

.github/actions/rust-toolchain@stable/action.yml

@@ -0,0 +1,22 @@
+name: Stable Rust Toolchain
+
+on:
+  workflow_call:
+    inputs:
+      targets:
+        description: Comma-separated list of target triples to install for this toolchain
+        required: false
+      components:
+        description: Comma-separated list of components to be additionally installed
+        required: false
+
+permissions:
+  contents: read
+
+runs:
+  using: "composite"
+  steps:
+    - uses: dtolnay/rust-toolchain@stable
+      with:
+        targets: ${{ inputs.targets }}
+        components: ${{ inputs.components }}