Bug 1302704 - part 4 - provide a target directory for gkrust and gkrust-gtest; r=chmanchester

Doing this gets us all of the benefits that we want from Cargo
workspaces (only compiling artifacts from gkrust-shared one time for
both libraries) without all the hassles that using Cargo workspaces
would require.  We don't have to worry about multiple Cargo processes
running simultaneously and racing to write files; Cargo is smart enough
to lock the build directory to serialize access to it.

toolkit/library/gtest/rust/moz.build

@@ -14,4 +14,4 @@ if CONFIG['MOZ_STYLO']:
 if CONFIG['MOZ_ENABLE_WEBRENDER']:
     features += ['quantum_render']
 
-RustLibrary('gkrust-gtest', features)
+RustLibrary('gkrust-gtest', features, '../..')

toolkit/library/rust/moz.build

@@ -14,4 +14,4 @@ if CONFIG['MOZ_STYLO']:
 if CONFIG['MOZ_ENABLE_WEBRENDER']:
     features += ['quantum_render']
 
-RustLibrary('gkrust', features)
+RustLibrary('gkrust', features, '..')