Meson build: build dir is hardcoded in sage.config

[antonio-rojas]

Steps To Reproduce

sage: from sage.env import SAGE_ROOT
sage: SAGE_ROOT
'/build/sagemath-git/src/sage/src/sage/../..'

SAGE_ROOT should be empty in sagelib except if explicitly specified (eg. by sage-the-distro). Otherwise things break badly for distro packages:

**********************************************************************
File "sage/src/sage/doctest/forker.py", line 1764, in sage.doctest.forker.DocTestDispatcher.serial_dispatch
Failed example:
    DC.expand_files_into_sources()
Exception raised:
    Traceback (most recent call last):
      File "/usr/lib/python3.13/site-packages/sage/doctest/forker.py", line 730, in _run
        self.compile_and_execute(example, compiler, test.globs)
        ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "/usr/lib/python3.13/site-packages/sage/doctest/forker.py", line 1154, in compile_and_execute
        exec(compiled, globs)
        ~~~~^^^^^^^^^^^^^^^^^
      File "<doctest sage.doctest.forker.DocTestDispatcher.serial_dispatch[8]>", line 1, in <module>
        DC.expand_files_into_sources()
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
      File "/usr/lib/python3.13/site-packages/sage/doctest/control.py", line 1048, in expand_files_into_sources
        self.sources = [FileDocTestSource(path, self.options) for path in expand()]
                                                                          ~~~~~~^^
      File "/usr/lib/python3.13/site-packages/sage/doctest/control.py", line 1045, in expand
        elif not skipfile(path, bool(self.options.optional),
                 ~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                        if_installed=self.options.if_installed, log=self.log):  # log when directly specified filenames are skipped
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "/usr/lib/python3.13/site-packages/sage/doctest/control.py", line 322, in skipfile
        with open(filename) as F:
             ~~~~^^^^^^^^^^
    FileNotFoundError: [Errno 2] No such file or directory: '/build/sagemath-git/src/sage/src/sage/../../src/sage/rings/homset.py'
**********************************************************************

Expected Behavior

SAGE_ROOT is unset (so expand_files_into_sources() works properly)

Actual Behavior

See above

Additional Information

No response

Environment

• OS: Arch Linux
• Sage Version: 10.7.beta0

Checklist

• I have searched the existing issues for a bug report that matches the one I want to file, without success.
• I have read the documentation and troubleshoot guide

[Krishnadubey1008]

@antonio-rojas please review the pr

[antonio-rojas]

The PR doesn't work, the env command reads the value of SAGE_ROOT from config.py, which is where the wrong value is set (also, setting it to the empty string wouldn't work anyway, it needs to be unset)

SAGE_ROOT is a sage-the-distro thing, it should not be set by sagelib. In fact, I don't think config.py should be installed by sagelib at all, it contains distribution-specific data that should be set by the distribution. sagelib should simply set sane defaults in env.py, as it has been doing so far.