integ-cli: also preserve SystemRoot env var

Windows CI fails to dial remote test host over tcp in the test cases where
we clear environment variables during `exec(dockerBinary, ...)` in the
absence of `SystemRoot` environment variable (typically points to `c:\windows`).

This fixes tests:
- `TestRunEnvironmentErase`
- `TestRunEnvironmentOverride`

Signed-off-by: Ahmet Alp Balkan <ahmetalpbalkan@gmail.com>