Don't auto-update the Python version in conda GHA tests

Author: blnicho

.github/workflows/test_branches.yml

@@ -373,7 +373,7 @@ jobs:
         fi
         # Note: this will fail the build if any installation fails (or
         # possibly if it outputs messages to stderr)
-        conda install --update-deps -q -y $CONDA_DEPENDENCIES
+        conda install --update-deps -q -y python="${{matrix.python}}" $CONDA_DEPENDENCIES
         if test -z "${{matrix.slim}}"; then
             # xpress.init() (xpress 9.5.1 from conda) hangs indefinitely
             # on GHA/Windows under Python 3.10 and 3.11.  Exclude that

.github/workflows/test_pr_and_main.yml

@@ -425,7 +425,7 @@ jobs:
         fi
         # Note: this will fail the build if any installation fails (or
         # possibly if it outputs messages to stderr)
-        conda install --update-deps -q -y $CONDA_DEPENDENCIES
+        conda install --update-deps -q -y python="${{matrix.python}}" $CONDA_DEPENDENCIES
         if test -z "${{matrix.slim}}"; then
             # xpress.init() (xpress 9.5.1 from conda) hangs indefinitely
             # on GHA/Windows under Python 3.10 and 3.11.  Exclude that