Rollup merge of #95441 - AlecGoncharow:issue-95204-fix, r=Mark-Simulacrum

Always use system `python3` on MacOS

This PR includes 2 changes:

1. Always use the system Python found at `/usr/bin/python3` on MacOS
2. Removes the hard requirement on having `python` in your system path if you didn't specify alternatives. The proposed change will instead attempt to find and use in order: `python` -> `python3` -> `python2`. This change isn't strictly necessary but without any change to this check, the original issue inspiring this change will still exist.

Fixes #95204
r? ```@jyn514```

src/bootstrap/lib.rs

@@ -1176,7 +1176,17 @@ impl Build {
 
     /// Path to the python interpreter to use
     fn python(&self) -> &Path {
-        self.config.python.as_ref().unwrap()
+        if self.config.build.ends_with("apple-darwin") {
+            // Force /usr/bin/python3 on macOS for LLDB tests because we're loading the
+            // LLDB plugin's compiled module which only works with the system python
+            // (namely not Homebrew-installed python)
+            Path::new("/usr/bin/python3")
+        } else {
+            self.config
+                .python
+                .as_ref()
+                .expect("python is required for running LLDB or rustdoc tests")
+        }
     }
 
     /// Temporary directory that extended error information is emitted to.