gh-39885: Provided a default value for SAGE_ROOT that can be overridden by an environment variable or configuration file
    
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This PR fixes #39870
added following code to `env.py`
```
DEFAULT_SAGE_ROOT = ""
SAGE_ROOT = var("SAGE_ROOT", DEFAULT_SAGE_ROOT)
```
to ensure that `SAGE_ROOT` is not hardcoded and can be dynamically set
or overridden
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URL: #39885
Reported by: Krishna Dubey
Reviewer(s): Antonio Rojas, Tobias Diez

Author: Release Manager

src/sage/env.py

@@ -192,7 +192,7 @@ def var(key: str, *fallbacks: Optional[str], force: bool = False) -> Optional[st
 SAGE_SPKG_INST = var("SAGE_SPKG_INST", join(SAGE_LOCAL, "var", "lib", "sage", "installed"))  # deprecated
 
 # source tree of the Sage distribution
-SAGE_ROOT = var("SAGE_ROOT")  # no fallback for SAGE_ROOT
+SAGE_ROOT = var("SAGE_ROOT") or None
 SAGE_SRC = var("SAGE_SRC", join(SAGE_ROOT, "src"), SAGE_LIB)
 SAGE_DOC_SRC = var("SAGE_DOC_SRC", join(SAGE_ROOT, "src", "doc"), SAGE_DOC)
 SAGE_PKGS = var("SAGE_PKGS", join(SAGE_ROOT, "build", "pkgs"))

src/sage/meson.build

@@ -4,7 +4,6 @@ sage_install_dir = py.get_install_dir() / 'sage'
 # Generate the configuration file
 conf_data = configuration_data()
 conf_data.set('PACKAGE_VERSION', '1.2.3')
-conf_data.set('SAGE_ROOT', meson.current_source_dir() / '..' / '..')
 # We use Python's prefix here to make it work with conda
 prefix = fs.as_posix(py.get_variable('prefix', ''))
 conf_data.set('prefix', prefix)