Fix test that calls setenv() without synchronization

[mcy]

This test is flaky under the following interleaving:

• exec_to_string() grabs MUTATE_ENV.
• env_inherit_set() calls temp_env_var() which calls env::set_var().
• exec_to_string() executes some asserts, one of which fails because the other test messed with the environment in a way exec_to_string() did not expect.
• exec_to_string() panics and poisons MUTATE_ENV.
• env_inherit_set() grabs the lock and panics because it's poisoned.

Taking the lock before setenv fixes this.