utils: Squash SyntaxWarning with Python 3.12

When running with Python 3.12, the following warnings are emitted. Let's
fix them.

utils/config.py:240: SyntaxWarning: invalid escape sequence '\$'
utils/config.py:241: SyntaxWarning: invalid escape sequence '\$'
utils/config.py:242: SyntaxWarning: invalid escape sequence '\{'
utils/config.py:245: SyntaxWarning: invalid escape sequence '\}'

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>