bash unset modifies the current shell's environment but the act_runner
at 172.0.0.29 re-injects runner-level environment variables before each
cargo invocation, making unset ineffective. env -u removes the variable
from the child process's environment at the exec() level, bypassing any
runner re-injection entirely.
Applies to all three non-macOS platforms (linux-amd64, windows-amd64,
linux-arm64) in both release-beta.yml and auto-tag.yml.
