Bug 1580689 - [geckodriver] Build linux32 geckodriver binaries via toolchain build task. r=nalexander

Differential Revision: https://phabricator.services.mozilla.com/D47109

--HG--
extra : moz-landing-system : lando

taskcluster/ci/config.yml

@@ -105,6 +105,7 @@ treeherder:
         'Nexus 5-L': 'Nexus 5-L Device Image'
         'I': 'Docker Image Builds'
         'TL': 'Toolchain builds for Linux 64-bits'
+        'TL32': 'Toolchain builds for Linux 32-bits'
         'TM': 'Toolchain builds for OSX'
         'TMW': 'Toolchain builds for Windows MinGW'
         'TW32': 'Toolchain builds for Windows 32-bits'

taskcluster/ci/toolchain/geckodriver.yml

@@ -19,6 +19,15 @@ job-defaults:
             # Changes to 'third_party/rust' should be captured by
             # 'testing/geckodriver/Cargo.toml'.
 
+linux32-geckodriver:
+    treeherder:
+        symbol: TL32(geckodriver)
+    run:
+        arguments: ['i686-unknown-linux-gnu']
+    fetches:
+        toolchain:
+            - linux64-rust
+
 linux64-geckodriver:
     treeherder:
         symbol: TL(geckodriver)