Use .env file instead of .envrc (astral-sh#1132)

astral-sh#1131 shows that `direnv` installation is _most_ of the CI overhead
introduced by astral-sh#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).

.env

@@ -0,0 +1,2 @@
+PATH=$PWD/bin:$PATH
+PUFFIN_PYTHON_PATH=$PWD/bin

.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:

CONTRIBUTING.md

@@ -43,21 +43,20 @@ Install required Python versions with the bootstrapping script:
 scripts/bootstrap/install.sh
 ```
 
-Then add the Python binaries to your path:
+The installed Python binaries will be available in `<repo>/bin` and must be added to your path to be used. We
+provide a `.env` file with the proper environment variables for development. You may activate it with:
 
 ```
-export PATH=$PWD/bin:$PATH
+source .env
 ```
 
-We also strongly recommend setting the `PUFFIN_PYTHON_PATH` variable during development; this will prevent your
-system Python versions from being found during tests:
+Or, if you use `direnv` to manage your environment:
 
 ```
-export PUFFIN_PYTHON_PATH=$PWD/bin
+echo "dotenv" >> .envrc
+direnv allow
 ```
 
-If you use [direnv](https://direnv.net/), these variables will be exported automatically after you run `direnv allow`.
-
 ## Testing
 
 To run the tests we recommend [nextest](https://nexte.st/). Make sure to run the tests with `--all-features`, otherwise you'll miss most of our integration tests.