T25: CI's cache step runs only in the Linux container: on the owner's own macOS and Windows runners its post step cleaned their real ~/.cargo, deleting rustup after every job and leaving its proxies pointing at nothing

.github/workflows/ci.yml

@@ -14,7 +14,6 @@ jobs:
         with:
           toolchain: "1.95"
           components: clippy, rustfmt
-      - uses: Swatinem/rust-cache@v2
       - run: scripts/check.sh
       - run: cargo build --locked
 
@@ -32,7 +31,9 @@ jobs:
       - uses: dtolnay/rust-toolchain@master
         with:
           toolchain: "1.95"
-      - uses: Swatinem/rust-cache@v2
+      # On a host runner its post step deletes the host's rustup.
+      - if: runner.os == 'Linux'
+        uses: Swatinem/rust-cache@v2
       - if: runner.os != 'Linux'
         run: cargo test --locked --quiet
       # The Linux job container runs as root, which ignores the file permissions the tests check.