[geary] build: Fix Flatpak CI FTB with GMime mainline

[Michael Gratton <mjog src gnome org>]

commit c0577d4bf64c3ae3aa6a0b14028d702d5ba895d4
Author: Michael Gratton <mike vee net>
Date:   Thu Apr 22 22:39:16 2021 +1000

    build: Fix Flatpak CI FTB with GMime mainline
    
    See !699 and https://github.com/jstedfast/gmime/pull/101

 org.gnome.Geary.json | 2 +-
 org.gnome.Geary.yaml | 4 +++-
 2 files changed, 4 insertions(+), 2 deletions(-)
---
diff --git a/org.gnome.Geary.json b/org.gnome.Geary.json
index 5c983803e..b65c3d49e 100644
--- a/org.gnome.Geary.json
+++ b/org.gnome.Geary.json
@@ -224,7 +224,7 @@
                 {
                     "type": "git",
                     "url": "https://github.com/jstedfast/gmime.git";,
-                    "branch": "master"
+                    "branch": "3.2.7"
                 }
             ]
         },
diff --git a/org.gnome.Geary.yaml b/org.gnome.Geary.yaml
index 908ed45db..f47a86d94 100644
--- a/org.gnome.Geary.yaml
+++ b/org.gnome.Geary.yaml
@@ -223,7 +223,9 @@ modules:
     sources:
       - type: git
         url: https://github.com/jstedfast/gmime.git
-        branch: master
+        # Pin to 3.2.7 until 3.2.8 is released, or the mainline branch
+        # is pre-bumped to same.
+        branch: 3.2.7
 
   # Geary dependency
   - name: libpeas