Don't rely on PATH to find sphinx-build

This change makes it so that we don't rely on the correct `sphinx-build`
being on PATH to build the docs.

This is useful when the command that runs sphinx-autobuild runs in a
virtualenv that isn't the currently activated virtualenv (i.e. via `uv
run --extra docs` invocation)

sphinx_autobuild/build.py

@@ -1,6 +1,7 @@
 """Logic for interacting with sphinx-build."""
 
 import subprocess
+import sys
 
 from sphinx_autobuild.utils import show
 
@@ -28,9 +29,12 @@ def __call__(self, *, rebuild: bool = True):
             )
             raise
 
-        show(command=["sphinx-build"] + self.sphinx_args)
+        show(command=["python", "-m", "sphinx"] + self.sphinx_args)
         try:
-            subprocess.run(["sphinx-build"] + self.sphinx_args, check=True)
+            subprocess.run(
+                [sys.executable, "-m", "sphinx"] + self.sphinx_args,
+                check=True,
+            )
         except subprocess.CalledProcessError as e:
             print(f"Sphinx exited with exit code: {e.returncode}")
             print(