Performance test script: speed up build, support Nitro

[tautschnig]

The first commit might also be of interest to the CI set-up as similar changes could/should be made to the CodeBuild configuration.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[tautschnig]

@kroening Please take a look as there is may be of interest to speed up CodeBuild jobs.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: abf7258).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/95301548

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 0338a13).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/95581623

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 24a31ad).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/95719983

[kroening]

@tautschnig Yes, I will give that a try.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: c37290f).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/95863273