osx: explicitly build hg with /usr/bin/python2.7

This should help avoid creating a package that depends on a custom
Python, as happened when I built a package for 3.8.

Makefile

@@ -156,7 +156,7 b' i18n/hg.pot: $(PYFILES) $(DOCFILES) i18n'
 # Packaging targets
 
 osx:
-	python setup.py install --optimize=1 \
+	/usr/bin/python2.7 setup.py install --optimize=1 \
 	  --root=build/mercurial/ --prefix=/usr/local/ \
 	  --install-lib=/Library/Python/2.7/site-packages/
 	make -C doc all install DESTDIR="$(PWD)/build/mercurial/"

tests/test-mac-packages.t

@@ -43,5 +43,11 b' Spot-check some randomly selected files:'
   $ grep '/hg	' boms.txt | cut -d '	' -f 1,2,3
   ./usr/local/bin/hg	100755	0/0
 
+Make sure the built binary uses the system Python interpreter
+  $ bsdtar xf mercurial.pkg/Payload usr/local/bin
+Use a glob to find this to avoid check-code whining about a fixed path.
+  $ head -n 1 usr/local/b?n/hg
+  #!/System/Library/Frameworks/Python.framework/Versions/2.7/Resources/Python.app/Contents/MacOS/Python
+
 Note that we're not currently installing any /etc/mercurial stuff,
 including merge-tool configurations.