Use CARGO env variable if present (#453)

* Run `$CARGO -vV` in `detect-targets` if env `CARGO` present
* Improve crate doc for `detect-targets`
* Use env var `CARGO` in `install_from_source` if present
* Test use of `CARGO` env in `tests.sh`

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

.github/workflows/integration.yml

@@ -51,6 +51,4 @@ jobs:
     - name: Test
       shell: bash
       run: .github/scripts/tests.sh ${{ matrix.bin }} ${{ runner.os }}
-      env:
-        CARGO_HOME: /tmp/cargo-home-for-test/