slapgrid: Fix instance python executable inspection

The path of the instance's python executable is inferred from the
shebang of ~/software_release/bin/buildout.

But when the path is too long, buildout generates a workaround
shebang using /bin/sh to exec the actual python executable.

slapos/grid/utils.py

@@ -33,6 +33,7 @@ import hashlib
 import os
 import pkg_resources
 import pwd
+import shlex
 import stat
 import sys
 import threading
@@ -177,10 +178,18 @@ def getPythonExecutableFromSoftwarePath(software_path):
   try:
     with open(os.path.join(software_path, 'bin', 'buildout')) as f:
       shebang = f.readline()
+      line = f.readline()
   except (IOError, OSError):
     return
   if shebang.startswith('#!'):
-    return shebang[2:].split(None, 1)[0]
+    executable = shebang[2:].split(None, 1)[0]
+    if executable.startswith('/bin/sh'):
+      # workaround for workaround for shebang length limitation:
+      # https://lab.nexedi.com/nexedi/slapos.buildout/blob/00d423bd76aa53a02aeaa7c593138b7396ffee1a/src/zc/buildout/easy_install.py#L172
+      args = shlex.split(line)
+      if len(args) >= 2 and args[0] == 'exec':
+        return args[1]
+    return executable
 
 
 def getCleanEnvironment(logger, home_path='/tmp'):