Hard code python interpreter

[furtib]

Why:
We want to support Bazel 8.
This is a temporary solution to solve toolchain issues.

What:

• Added shebang #!/usr/bin/env python3 to codechecker_script.py and per_file_script.py.
• Removed dead code from common.bzl

Addresses:
#108

Notes:
Bazel 8 support will be a chain of small patches, of which this is the first.
Other things necessary for Bazel 8: (As demonstrated in #203)

• Add rules_python in MODULE.bazel. (This should be a one-liner, only needed for py_binary targets)
• Apply patch Extend external test to Bazel 8 #159 and Bazel 8 specific miscellanious changes #161.