utils: Move flags into re.compile statements

[RedL0tus]

re in Python 3.14 no longer allows setting flags with compiled statements.

Example error log:

[ERROR]: cannot process flags argument with a compiled pattern
Traceback (most recent call last):
  File "/usr/lib/python3.14/site-packages/acbs/main.py", line 213, in build
    self.build_sequential(build_timings, packages)
    ~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.14/site-packages/acbs/main.py", line 315, in build_sequential
    write_checksums(spec_location, checksum)
    ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.14/site-packages/acbs/utils.py", line 422, in write_checksums
    if re.search(chksum_pattern, content, re.MULTILINE | re.DOTALL):
       ~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.14/re/__init__.py", line 177, in search
    return _compile(pattern, flags).search(string)
           ~~~~~~~~^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.14/re/__init__.py", line 345, in _compile
    raise ValueError(
        "cannot process flags argument with a compiled pattern")
ValueError: cannot process flags argument with a compiled pattern
[INFO]: ACBS is trying to save your build status...
[INFO]: ... saved to /tmp/1778094153.acbs-ckpt

[CRIT]: Oops! RuntimeError: Build error.
Use `acbs-build --resume /tmp/1778094153.acbs-ckpt` to resume after you sorted out the situation.

[what-the-diff]

PR Summary

• Expanded Matching Capabilities in Pattern Regex Compilations
  The update adds flags re.MULTILINE | re.DOTALL to the chksum_pattern and tarball_pattern regular expressions. This enhances their compiling process, boosting the ability to match patterns across different lines and to consider all characters in the process.

• Default Matching in write_checksums Function
  The update also removes flags from the re.search and re.sub calls within the write_checksums function. This reversion means the function now uses default matching behaviour, which simplifies the overall code execution.