Temporarily break pure-rust-build to verify that it can fail

This makes the `pure-rust-build` CI job attempt to install gitoxide
from `.` with implicit `max`, rather than explicit `max-pure`.

This job, introduced in ed4deac (#624), is intended to identify
unintended C toolchain dependencies. If, without the current
breakage, it is (still) capable of doing this, then as temporarily
modified here, it should fail. If it fails and the error messages
clearly relate to the absence of components necessary to build/use
C libraries, then it is probably (still) working.

Assuming it fails as expected, the Debian version can then be
increased, and then the changes here (and in any follow-up testing
adjustments) undone: if that fails, then succeeds, respectively,
then the job's functionality is most likely preserved across the
upgrade.

Author: EliahKagan

.github/workflows/ci.yml

@@ -28,7 +28,7 @@ jobs:
       - name: install Rust via Rustup
         run: curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --profile minimal;
       - uses: Swatinem/rust-cache@v2
-      - run: /github/home/.cargo/bin/cargo install --debug --locked --no-default-features --features max-pure --path .
+      - run: /github/home/.cargo/bin/cargo install --debug --locked --path .
 
   test:
     runs-on: ubuntu-latest