k9: use gradle 5.6.4 until 6.0.1 is on the buildserver

cketti said it'll be fine :-).

metadata/com.fsck.k9.yml

@@ -482,7 +482,9 @@ Builds:
     output: app/k9mail/build/outputs/apk/release/k9mail-release-unsigned.apk
     rm:
       - tools/android-studio/settings.jar
-    prebuild: echo 'org.gradle.jvmargs=-Xmx1g' >> ../../gradle.properties
+    prebuild:
+      - echo 'org.gradle.jvmargs=-Xmx1g' >> gradle.properties
+      - sed -i -e 's/6.0.1/5.6.4/' gradle/wrapper/gradle-wrapper.properties
 
 MaintainerNotes: |-
     Use output instead of subdir to make gradlew-fdroid correctly determine the gradle version to be used.