If you compile SwiftShader with gcc and the --coverage flag, the build
will contain coverage instrumentation. However, the coverage
instrumentation races when multiple instances of SwiftShader run in
parallel. This change makes Regres replace the string "PROC_ID" inside
the GCOV_PREFIX environment variable with an ID that is unique between
parallel dEQP processes. This allows the user to prevent races using
e.g. GCOV_PREFIX=/tmp/cov/PROC_ID

Change-Id: I227896053c5a0ea34dc8acc282014970a626785d
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38928Reviewed-by: Ben Clayton <bclayton@google.com>
Tested-by: Paul Thomson <paulthomson@google.com>