ci: Avoid OpenWatcom linker temp file collisions

When wlink needs to spill into a temporary file, code in spillio.c checks environment variables WLINKTMP, TMP, and TMPDIR to get a directory for temporary files. It searches for a name WLK02112.xx* that doesn't exist, where * starts at a and increments. Then it opens the file with the chosen name. This can race among concurrent wlink invocations that may all chose the same name before opening, and causes errors like:

Error! E3008: cannot open C:\...\Temp\WLK02112.xxa : Permission denied

Since wmake does not run parallel jobs, this normally isn't a problem. However, our test suite runs multiple tests in parallel. Each test has its own wmake invocation, and therefore can run wlink concurrently.

Set WLINKTMP=. in the environment to tell each wlink to place temporary files in its own working directory, which will be different for each separate wmake invocation in our test suite.

See this code in the Open Watcom V2 Fork for the temp directory selection and temp file name race.