build: make per-build hard time limit customizable

Add "timeout=n" metadata field that overrides build timeout (in seconds).
The default is 7200, i.e. 2 hours.

tests/metadata/dump/com.politedroid.yaml

@@ -71,6 +71,7 @@ builds:
   submodules: false
   sudo: ''
   target: android-10
+  timeout: null
   versionCode: '3'
   versionName: '1.2'
 - androidupdate: []
@@ -104,6 +105,7 @@ builds:
   submodules: false
   sudo: ''
   target: android-15
+  timeout: null
   versionCode: '4'
   versionName: '1.3'
 - androidupdate: []
@@ -137,6 +139,7 @@ builds:
   submodules: false
   sudo: ''
   target: android-10
+  timeout: null
   versionCode: '5'
   versionName: '1.4'
 - androidupdate: []
@@ -171,6 +174,7 @@ builds:
   submodules: false
   sudo: echo 'this is just a test'
   target: null
+  timeout: null
   versionCode: '6'
   versionName: '1.5'
 comments: {}