fix MSRV check on Windows

turns out that Powershell is used by default there, and variable substitutions
are different there.
What's worse is that it doesn't seem to fail if something doesn't work as one
would expect.

Author: Byron

.github/workflows/msrv.yml

@@ -25,6 +25,6 @@ jobs:
       - uses: actions/checkout@v4
       - uses: extractions/setup-just@v2
       - run: |
-          rustup toolchain install $rust_version --profile minimal --no-self-update
-          rustup default $rust_version
+          rustup toolchain install ${{ env.rust_version }} --profile minimal --no-self-update
+          rustup default ${{ env.rust_version }}
       - run: just ci-check-msrv