Add support for IA_CONFIG_FILE environment variable

ia-upload-stream

@@ -41,15 +41,19 @@ class PreventCompletionError(UploadError):
 
 def get_ia_access_secret(configFile = None):
 	if configFile is None:
-		# This part of the code is identical (except for style changes) to the one in internetarchive and was written from scratch by JustAnotherArchivist in May 2021.
+		# This part of the code is identical (except for style changes) to the one in internetarchive and was written from scratch by JustAnotherArchivist in May and December 2021.
+		candidates = []
+		if os.environ.get('IA_CONFIG_FILE'):
+			candidates.append(os.environ['IA_CONFIG_FILE'])
 		xdgConfigHome = os.environ.get('XDG_CONFIG_HOME')
 		if not xdgConfigHome or not os.path.isabs(xdgConfigHome) or not os.path.isdir(xdgConfigHome):
 			# Per the XDG Base Dir specification, this should be $HOME/.config. Unfortunately, $HOME does not exist on all systems. Therefore, we use ~/.config here.
 			# On a POSIX-compliant system, where $HOME must always be set, the XDG spec will be followed precisely.
 			xdgConfigHome = os.path.join(os.path.expanduser('~'), '.config')
-		for candidate in [os.path.join(xdgConfigHome, 'internetarchive', 'ia.ini'),
-		                  os.path.join(os.path.expanduser('~'), '.config', 'ia.ini'),
-		                  os.path.join(os.path.expanduser('~'), '.ia')]:
+		candidates.append(os.path.join(xdgConfigHome, 'internetarchive', 'ia.ini'))
+		candidates.append(os.path.join(os.path.expanduser('~'), '.config', 'ia.ini'))
+		candidates.append(os.path.join(os.path.expanduser('~'), '.ia'))
+		for candidate in candidates:
 			if os.path.isfile(candidate):
 				configFile = candidate
 				break