Regtest cmake build.log overwritten

[ukmo-ccbunney]

Describe the bug
When the regression tests are running their CMake builds, the build.log for the shared build is overwritten when the MPI build is run. This means we lose any compiler messages/warnings from the shared build.

To Reproduce
Run any MPI enabled regression test

Expected behavior
If you run tail -f build.log when the build is running, you will see the compiler output for the shared build.
When the MPI build starts, this output will be overwritten (tail will report the file as being truncated).