Add e2e test via Github workflow

In #1036, an issue was reported which reported toolchain install issues, possibly caused by path separator issues on windows (ubuntu and mac seem to be fine).

With this commit, we add a run of cargo-msrv as an e2e test to our own CI on windows and mac while we're at it.
Previously, we would only run on ubuntu; the e2e test was a side-effect of checking our own MSRV.
We do have tests which run via the 'cargo run' vehicle and temporary directories, but that's not as sandboxed as this approach.

Hopefully, running all primary platforms will (1) give an indication of whether #1036 is indeed a cargo-msrv issue, and (2) prevent similar bugs in the future.

.github/workflows/msrv.yml

@@ -4,22 +4,40 @@ on:
   push:
     branches:
       - master
-      - main
+      - mainLOCAL_URL
   merge_group:
   schedule:
     - cron: '00 06 * * *'
 jobs:
+  # MSRV check and e2e test
   msrv:
     name: msrv
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        build: [ubuntu, macos, windows]
+        include:
+          - build: ubuntu
+            os: ubuntu-latest
+
+          - build: macos
+            os: macos-13
+
+          - build: windows
+            os: windows-latest
     continue-on-error: true
     steps:
       - name: checkout_repo
         uses: actions/checkout@v4
       - name: install_rust
         uses: dtolnay/rust-toolchain@stable
       - name: install_cargo_msrv
-        run: cargo install cargo-msrv --all-features
+        if: matrix.build == 'ubuntu'
+        run: cargo install cargo-msrv
+      - name: install_cargo_msrv_no_default
+        if: matrix.build != 'ubuntu'
+        run: cargo install cargo-msrv --no-default-features
       - name: version_of_cargo_msrv
         run: cargo msrv --version
       - name: run_cargo_msrv