Use xcode 9.3 on all osx builders

Author: alexcrichton

.azure-pipelines/steps/run.yml

@@ -35,11 +35,14 @@ steps:
   displayName: Install build dependencies (OSX)
   condition: and(succeeded(), eq(variables['Agent.OS'], 'Darwin'), eq(variables['RUST_CHECK_TARGET'],'dist'))
 
+# Switch to XCode 9.3 on OSX since it seems to be the last version that supports
+# i686-apple-darwin. We'll eventually want to upgrade this and it will probably
+# force us to drop i686-apple-darwin, but let's keep the wheels turning for now.
 - bash: |
     set -e
     sudo xcode-select --switch /Applications/Xcode_9.3.app
   displayName: Switch to Xcode 9.3 (OSX)
-  condition: and(succeeded(), eq(variables['Agent.OS'], 'Darwin'), eq(variables['RUST_CHECK_TARGET'],'dist'))
+  condition: and(succeeded(), eq(variables['Agent.OS'], 'Darwin'))
 
 - template: install-windows-build-deps.yml