[services] Clean up install_cryptopp.sh using ShellCheck

Summary: Related Linear issue here. This is part of a set of diffs that will allow ShellCheck to be added to the CI. See inline comments for specific details of the ShellCheck error/warning output.

Test Plan:
ShellCheck

This script is run in the tunnelbroker Docker build, so if that CI continues to build with no errors we can reasonably assume this script continues to work as expected.

Also, I ran git grep 'CXXFLAGS' and found no occurrences of it in the codebase.

Reviewers: atul, jon, tomek, karol

Reviewed By: atul, jon

Subscribers: ashoat, adrian, karol

Differential Revision: https://phab.comm.dev/D4738