Use .env file instead of .envrc (#1132)

#1131 shows that `direnv` installation is _most_ of the CI overhead
introduced by #1105.

Instead of using `direnv`, let's just use a simple `.env` file that can
be loaded with `source` or [`direnv`'s `dotenv`
directive](https://direnv.net/man/direnv-stdlib.1.html#codedotenv-ltdotenvpathgtcode).

.github/workflows/ci.yml

@@ -56,9 +56,7 @@ jobs:
       - uses: actions/checkout@v4
       - name: "Install required Python versions"
         run: |
-          sudo apt install direnv
           scripts/bootstrap/install.sh
-          direnv allow .envrc
       - name: "Install Rust toolchain"
         run: rustup show
       - uses: rui314/setup-mold@v1
@@ -71,7 +69,8 @@ jobs:
           save-if: ${{ github.ref == 'refs/heads/main' }}
       - name: "Tests"
         run: |
-          direnv exec . cargo nextest run --all --all-features --status-level skip --failure-output immediate-final --no-fail-fast -j 12
+          source .env
+          cargo nextest run --all --all-features --status-level skip --failure-output immediate-final --no-fail-fast -j 12
 
   # TODO(konstin): Merge with the cargo-test job once the tests pass
   windows: