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.

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