scripts: Fix exec invocation for in-process virtualenv activation.

activate_this.py has always documented that it should be exec()ed with
locals = globals, and in virtualenv 16.0.0 it raises a NameError
otherwise.

As a simplified demonstration of the weird things that can go wrong
when locals ≠ globals:

    >>> exec('a = 1; print([a])', {}, {})
    [1]
    >>> exec('a = 1; print([a for b in [1]])', {}, {})
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "<string>", line 1, in <module>
      File "<string>", line 1, in <listcomp>
    NameError: name 'a' is not defined
    >>> exec('a = 1; print([a for b in [1]])', {})
    [1]

Top-level assignments go into locals, but from inside a new scope like
a list comprehension, they’re read out of globals, which doesn’t work.

Fixes #12030.

Signed-off-by: Anders Kaseorg <anders@zulipchat.com>

tools/lib/provision.py

@@ -393,7 +393,7 @@ def main(options):
     setup_venvs.main()
 
     activate_this = "/srv/zulip-py3-venv/bin/activate_this.py"
-    exec(open(activate_this).read(), {}, dict(__file__=activate_this))
+    exec(open(activate_this).read(), dict(__file__=activate_this))
 
     setup_shell_profile('~/.bash_profile')
     setup_shell_profile('~/.zprofile')