build_release: Separate 32/64-bit pyinstaller envs on Windows

For some reason, pip fails now when trying to "update" the existing source
clone...

scripts/dev/build_release.py

@@ -275,7 +275,7 @@ def build_windows():
 
     utils.print_title("Running pyinstaller 32bit")
     _maybe_remove(out_32)
-    call_tox('pyinstaller', '-r', python=python_x86)
+    call_tox('pyinstaller32', '-r', python=python_x86)
     shutil.move(out_pyinstaller, out_32)
 
     utils.print_title("Running pyinstaller 64bit")

tox.ini

@@ -167,6 +167,17 @@ deps =
 commands =
     {envbindir}/pyinstaller --noconfirm misc/qutebrowser.spec
 
+[testenv:pyinstaller32]
+# A copy of the pyinstaller environment above, to be used for 32-bit on Windows
+# to make sure the both installations are separated.
+# This doesn't actually do anything 32-bit specific, that part happens by
+# setting the PYTHON environment variable accordingly.
+basepython = {[testenv:pyinstaller]basepython}
+pip_version = {[testenv:pyinstaller]pip_version}
+passenv = {[testenv:pyinstaller]passenv}
+deps = {[testenv:pyinstaller]deps}
+commands = {[testenv:pyinstaller]commands}
+
 [testenv:eslint]
 # This is duplicated in travis_run.sh for Travis CI because we can't get tox in
 # the JavaScript environment easily.