llvm-coverage: increase the CLANG_TOOLCHAIN_PROGRAM_TIMEOUT to 120s (#966)

This should decrease the flakiness of tests like
clang/test/Driver/openmp-system-arch.c in this job, which has been
failing sporadically:

https://ci.swift.org/job/llvm.org/job/llvm-coverage/449/
https://ci.swift.org/job/llvm.org/job/llvm-coverage/443/

The value picked here maches what we've currently got in llvm-project's
github pre-merge workflow.

rdar://188173992