tox.ini: Use version of bst-plugins-experimental which doesn't try to test upstream git

As described here: https://gitlab.com/BuildStream/bst-plugins-experimental/-/merge_requests/175

tox.ini

diff --git a/tox.ini b/tox.ini
index e69efe8..2c80db4 100644
--- a/tox.ini
+++ b/tox.ini
@@ -8,7 +8,7 @@
 
 # Configuration variables to share across environments
 [config]
-BST_PLUGINS_EXPERIMENTAL_VERSION = 1.93.7
+BST_PLUGINS_EXPERIMENTAL_VERSION = 74d0a8d918563e70e6d140e0bc4765fbf4945dc5
 BST_PLUGINS_VERSION = ed65975aa3b0629a36112d474366ac86d4da8261
 
 #