exec(test): Do not clear PATH; this breaks coverage on Windows

[denik]

Changes

When setting up PATH in tests, put desired entry first but keep the rest as well. Otherwise it fails on Windows

D:/a/cli/cli/libs/exec/exec_test.go:108
Error: Received unexpected error:
exit status 0xc0000135

Explanation from Claude:

The error code 0xc0000135 is a Windows error indicating "Unable to locate DLL"
When code coverage is enabled, Go instruments the binary with coverage tracking code, which requires additional DLL dependencies on Windows.

Tests

Separate draft PR with coverage enabled on CI: #2141

[denik]

Integration tests failures are unrelated.