Update Briar headless to 1.2.7

* https://code.briarproject.org/briar/briar/-/wikis/changelog#anchor-127
* https://code.briarproject.org/briar/briar/-/tags/release-1.2.7
* https://media.dorfbrunnen.eu/briar/jars/009/

Fixes #13

app.briar.gtk.json

@@ -109,8 +109,8 @@
             "sources": [
                 {
                     "type": "file",
-                    "url": "https://media.dorfbrunnen.eu/briar/jars/007/briar-headless.jar",
-                    "sha256": "3dea594a88be91d2b40a61adcfdf03432dc9e3c2865317ccd7799e13321200ec"
+                    "url": "https://media.dorfbrunnen.eu/briar/jars/009/briar-headless.jar",
+                    "sha256": "63dc3c081b65b0e5edb5f154f33fa3b91f4ec8e76d6a1d1d9487b923d8a5f455"
                 }
             ]
         },