Bug 1467248 - Make the spidermonkey jobs labeled debug be debug builds, r=fitzgen

--HG--
extra : topic : nursery.strings
extra : rebase_source : 8ad004d6e7f4a1755f72063a571632fe7b18616d

taskcluster/ci/spidermonkey/linux.yml

@@ -29,7 +29,7 @@ sm-mozjs-sys-linux64/debug:
         symbol: SM(mozjs-crate)
     run:
         using: spidermonkey-mozjs-crate
-        spidermonkey-variant: plain
+        spidermonkey-variant: plaindebug
     run-on-projects: ['integration', 'release', 'try']
 
 sm-rust-bindings-linux64/debug:
@@ -44,7 +44,7 @@ sm-rust-bindings-linux64/debug:
         docker-image: {in-tree: debian7-mozjs-rust-build}
     run:
         using: spidermonkey-rust-bindings
-        spidermonkey-variant: plain
+        spidermonkey-variant: plaindebug
     run-on-projects: ['integration', 'release', 'try']
 
 sm-plain-linux64/debug: