[Bug] Fix bug to disable taichi header print (#8517)

Issue: #8334 

### Brief Summary

The previous code #8413 had no effect because the value in os.environ
can only be a string. This pull request will enable users to disable the
Taichi header print by setting one of the following ways:

- os.environ['ENABLE_TAICHI_HEADER_PRINT'] = 'False'
- os.environ['ENABLE_TAICHI_HEADER_PRINT'] = 'FALSE'
- os.environ['ENABLE_TAICHI_HEADER_PRINT'] = 'F'
- os.environ['ENABLE_TAICHI_HEADER_PRINT'] = '0'

### Walkthrough

copilot:walkthrough

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

python/taichi/_lib/utils.py

@@ -169,7 +169,7 @@ def _print_taichi_header():
     print(header)
 
 
-if os.environ.get("ENABLE_TAICHI_HEADER_PRINT", True):
+if os.getenv("ENABLE_TAICHI_HEADER_PRINT", "True").lower() not in ("false", "0", "f"):
     _print_taichi_header()