cleanup(ci): use bash colon builtin to set vars with defaults (#6722)
cleanup(ci): use bash colon builtin to set vars with defaults (#6722)

* cleanup(ci): use bash colon builtin to set vars with defaults

Uses the bash `:` builtin and `:=` in the parameter expansion to set
some bash variables with default values in a more succinct way.

See:
- https://www.gnu.org/savannah-checkouts/gnu/bash/manual/bash.html#Bourne-Shell-Builtins
- https://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html

* revert change to CODECOV_TOKEN and LOG_LINKER_PAT
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp