pyinstaller: embed capa version in exe

ci/pyinstaller.spec

@@ -1,7 +1,14 @@
 # -*- mode: python -*-
 import os.path
+import subprocess
+
 import wcwidth
 
+
+with open('./capa/version.py', 'wb') as f:
+    f.write("__version__ = '%s'"
+            % subprocess.check_output(["git", "describe", "--always"]).strip())
+
 a = Analysis(
     ['../capa/main.py'],
     pathex=['capa'],