Improve logging of warnings from Jinja2 lexing

[MetRonnie]

Follow-up to #7365

Changes the apparent source of some warnings raised during jinja2 lexing from the jinja2 source code to the Cylc config file

Repro

#!jinja2
{% set x = '\${HOME}/foo/bar' %}
[scheduler]
    allow implicit tasks = True
[scheduling]
    [[graph]]
        R1 = foo

Before

$ cylc validate wflow
WARNING - The following warnings were raised during Jinja2 preprocessing (note: any Jinja 3.1 deprecations will break at Cylc 8.7):
    .../lib/python3.14/site-packages/jinja2/lexer.py:654: DeprecationWarning: "\$" is an invalid escape sequence. Such sequences will not work in the future.
      .decode("unicode-escape")

After

$ cylc validate wflow
WARNING - The following warnings were raised during Jinja2 preprocessing (note: any Jinja 3.1 deprecations will break at Cylc 8.7):
    ~/cylc-run/wflow/flow.cylc:<jinja2 template>: DeprecationWarning: "\$" is an invalid escape sequence. Such sequences will not work in the future.

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• No dependency changes
• No tests included as minor
• Changelog entry not needed as follow-up to unreleased PR
• No docs needed
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.