ci: ditch unmaintained actions-rs/toolchain action

as mentioned in the previous commit, github is now issuing warnings for
ci actions that are using obsolete features. i managed to upgrade to using
a newer version of the `checkout` action very easily, however the rust
toolchain action that many people were using seems to have been
unmaintained for quite some time now with the author having gone awol.
some people suggest switching to an alternative by `dtolnay` (a rust dev?)
while others suggest just using `rustup` directly. i chose to do the
latter, and added a couple of suggested environment variables.

i also moved the rust/cargo version check into the same block now that it's
a sequence of commands rather than instigating an action helper.

.github/workflows/test.yml

@@ -58,11 +58,13 @@ jobs:
         uses: actions/checkout@v3
 
       - name: Install toolchain
-        uses: actions-rs/toolchain@v1
-        with:
-          profile: minimal
-          toolchain: ${{ matrix.rustc }}
-          override: true
+        run: |
+          rustup toolchain install ${{ matrix.rustc }} --profile minimal --no-self-update
+          rustup default ${{ matrix.rustc }}
+          echo CARGO_TERM_COLOR=always >> $GITHUB_ENV
+          echo RUST_BACKTRACE=1 >> $GITHUB_ENV
+          rustc -V
+          cargo -V
 
       - name: Install PulseAudio and `libpulse-dev`
         run: |
@@ -74,11 +76,6 @@ jobs:
           systemctl --user start pulseaudio
           systemctl --user status pulseaudio
 
-      - name: Check toolchain versions
-        run: |
-          cargo -V
-          rustc -V
-
       - name: Check building main binding (cargo check)
         working-directory: ./pulse-binding
         run: |