Nextest archive with --workspace-remap produces incorrect source file paths in reports

[yuvald-sweet-security]

• In the directory /tmp/test1, run cargo llvm-cov nextest-archive --archive-file /tmp/tests.tar.zst.
• Check out the source code to a new directory /tmp/test2, and use the nextest workspace-remap feature to specify that we're in a different directory: cargo llvm-cov --cobertura nextest --archive-file /tmp/tests.tar.zst --workspace-remap /tmp/test2
• The <source> tag shows /tmp/test2 correctly, but the source file names in the report are prefixed with /tmp/test1. This breaks various CI integrations because they can't find the source files in the report.