[ci][generate_cache] Run steps in parallel (#940)

In the shell script that is the main body of the `generate_cache` job,
background each task and then wait for them all to complete so that they
can run in parallel.

In one test, this sped up the execution time of the "Download
dependencies" step of the `generate_cache` job from 29s [1] to 18s [2].

[1] https://github.com/google/zerocopy/actions/runs/8022266381/job/21916187070?pr=932

[2] https://github.com/google/zerocopy/actions/runs/8022379583/job/21916562791?pr=940

.github/workflows/ci.yml

@@ -408,12 +408,17 @@ jobs:
           # there is no support for doing this directly [1], so we just check
           # all crates using --tests.
           #
+          # We background all jobs and then wait for them so that they can run
+          # in parallel.
+          #
           # [1]  https://stackoverflow.com/a/42139535/836390
-          cargo check --workspace --tests            &> /dev/null || true
-          cargo metadata                             &> /dev/null || true
-          cargo install cargo-readme --version 3.2.0 &> /dev/null || true
-          cargo install --locked kani-verifier       &> /dev/null || true
-          cargo kani setup                           &> /dev/null || true
+          cargo check --workspace --tests            &> /dev/null &
+          cargo metadata                             &> /dev/null &
+          cargo install cargo-readme --version 3.2.0 &> /dev/null &
+          cargo install --locked kani-verifier       &> /dev/null &
+          cargo kani setup                           &> /dev/null &
+
+          wait
 
   check-all-toolchains-tested:
     runs-on: ubuntu-latest