Merge branch 'willsalmon/backport1897' into bst-1

_downloadablefilesource.py: Set user agent

See merge request BuildStream/buildstream!1898
diff --git a/buildstream/plugins/sources/_downloadablefilesource.py b/buildstream/plugins/sources/_downloadablefilesource.py
index ab8140d..de12672 100644
--- a/buildstream/plugins/sources/_downloadablefilesource.py
+++ b/buildstream/plugins/sources/_downloadablefilesource.py
@@ -107,6 +107,7 @@
                 default_name = os.path.basename(self.url)
                 request = urllib.request.Request(self.url)
                 request.add_header('Accept', '*/*')
+                request.add_header('User-Agent', 'BuildStream/1')
 
                 # We do not use etag in case what we have in cache is
                 # not matching ref in order to be able to recover from