ci: let a cancellation reach the runtime adapter's suite #1004

#975 made the Linux steps that run test-microvm and the benchmarks exec their harness, so that the runner's SIGINT and SIGTERM reach it (#946).

The runtime adapter's Linux step in run-nvx-microvm-tests.yml (nvx.py test-runtime-adapter; earlier, cargo test) still runs in Bash's foreground. Its command doesn't pass a cancellation on to cargo test and the test binaries, so their VMs run until the runner's orphan cleanup.

To fix: exec the command, and have it forward SIGINT and SIGTERM to cargo test's process group, then wait for it.

Found while validating #963. Related to #833.