Bug when using certain characters in config file

[ddejong-therp]

I get this error when rebuilding waft where I have certain symbols in the common/conf.d/override-odoo.cfg file:

Traceback (most recent call last):
  File "/home/bamilab/code/waft14/.venv/bin/config-generate", line 61, in <module>
    result = Template(result).substitute(os.environ)
  File "/home/bamilab/code/waft14/.pyenv/versions/3.6.15/lib/python3.6/string.py", line 130, in substitute
    return self.pattern.sub(convert, self.template)
  File "/home/bamilab/code/waft14/.pyenv/versions/3.6.15/lib/python3.6/string.py", line 127, in convert
    self._invalid(mo)
  File "/home/bamilab/code/waft14/.pyenv/versions/3.6.15/lib/python3.6/string.py", line 103, in _invalid
    (lineno, colno))
ValueError: Invalid placeholder in string: line 14, col 19
Traceback (most recent call last):
  File "/home/bamilab/code/waft14/common/entrypoint", line 39, in <module>
    subprocess.check_call(command)
  File "/home/bamilab/code/waft14/.pyenv/versions/3.6.15/lib/python3.6/subprocess.py", line 311, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '/home/bamilab/code/waft14/common/entrypoint.d/50-config-generate' returned non-zero exit status 1.

Her is my current config file:

[options]
# Version: v.22.05.30
# Configure odoo options
# Feel free to tune the variables in this file as you like
# These variables here will override the variables in odoo.cfg
# Push this file to your own repository
without_demo =
db_name = odoo14
dbfilter = ^odoo14$

When I remove the ^ and $ symbols, it works again.
I've pulled the latest changes of waftlib master, to make sure it's still happening in the latest version.