Hack as flatdata::generate is not concurrency-safe

`flatdata::generate` (see: `lib/src/generator.rs`) uses `pip` to install
the flatdata generator. Concurrent calls to build it from source cause
it to fail.

Signed-off-by: Eike S R <eike.roehrs@gmail.com>

.github/workflows/rust.yml

@@ -21,12 +21,12 @@ jobs:
           # use generator from source
           export FLATDATA_GENERATOR_PATH=${PWD}/flatdata-generator
           cd flatdata-rs
-          cargo build --all-targets
-          cargo build --all-targets --all-features
+          cargo build --all-targets -j 1
+          cargo build --all-targets --all-features -j 1
       - name: Run tests
         run: |
           # use generator from source
           export FLATDATA_GENERATOR_PATH=${PWD}/flatdata-generator
           cd flatdata-rs
-          cargo test --all-targets
-          cargo test --all-targets --all-features
+          cargo test --all-targets -j 1
+          cargo test --all-targets --all-features -j 1