makefile: Clearer script calls

[doshitan]

Recommend avoiding backticks to indicate command substitution in general and to
standardize on $(). This can be a little confusing in a makefile which also
uses $() to indicate its own calls, but the consistency is better in my
opinion. So swap those where used.

Also, Make isn't Bash, so use explicit shell function to highlight what is
intended to be executed by Make.

Though CURRENT_ACCOUNT_ALIAS and CURRENT_ACCOUNT_ID also don't appear to be
used currently, so could also be dropped.

[rocketnova]

Appreciate your make expertise here.

What do you think is the best way to maintain these guidelines going forward? Different options I can think of, but that I am not tied to:

• Add a comment about these at the top of the file
• Add a Makefile styleguide to /docs
• Add a Makefile linter (does this exist / do you know any?)