Support executing fuzz tests with custom environment variables

## Requested Behavior
Specifying an `env` dictionary of environment variables in a `cc_fuzz_test` causes the regression test and fuzz test to execute with the environment variables set.

## Actual Behavior
Specifying an `env` dictionary of environment variables currently has no effect on the regression and fuzz tests.

## Motivation
We use environment variables to configure sanitizers (e.g., where to look for runtime suppressions, UBSan should crash upon finding UB, print a stack trace).

I'm presently working on extending rules_fuzzing to support `env`.