Add environment variable to redirect SYCL_PI_TRACE to file

**Is your feature request related to a problem? Please describe**
`SYCL_PI_TRACE` prints to a mix of `stdout` and `stderr`. If an application is reading from either of these streams, it may receive unexpected trace output (and break).

**Describe the solution you would like**
Add something like a `SYCL_PI_TRACE_FILE=pi_trace.txt` capability that can be used alongside `SYCL_PI_TRACE`.

**Describe alternatives you have considered**
Piping `stdout` or `stderr` to file doesn't work.

**Additional context**
N/A