Tests: Match expected environment modification more consistently

Revert the regex change from commit 01147454e7 (FASTBuild: Add
generator, 2025-07-30) and revise the build output to match anyway.
The actual problem was that build and install steps are driven through
a nested `cmake --build`, and `fbuild` prefixes the first line of build
output.  Add an innocuous first line so we can match the intended lines
consistently across generators.