SC2006: Use $(..) instead of legacy ...

Backtick command substitution STATEMENT is legacy syntax with several
issues.

• It has a series of undefined behaviors related to quoting in POSIX.
• It imposes a custom escaping mode with surprising results.
• It's exceptionally hard to nest.

$(STATEMENT) command substitution has none of these problems, and is
therefore strongly encouraged.

PR: 227109
Submitted by: mat
Sponsored by: Absolight