In nightly runs, make sure cargo is in $PATH (#462)

.github/workflows/nightly_tests.yml

@@ -33,6 +33,7 @@ jobs:
       run: |
         DEBIAN_FRONTEND=noninteractive sudo apt install -y --no-install-recommends curl
         curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- --default-toolchain nightly --profile minimal -y
+        source ~/.cargo/env
         pids=()
         exit_codes=()
         for i in $(seq 0 $((${{ env.TEST_THREADS }} - 1))); do