Update cargo.

Mark-Simulacrum · Mark-Simulacrum · commit 1b758fb6e35d · 2017-09-01T21:15:47.000-06:00
This includes rust-lang/cargo#4447 which fixes a bug in Cargo that is needed to fix rust-lang#44237.
diff --git a/src/tools/cargo b/src/tools/cargo
@@ -1 +1 @@
-Subproject commit 4b1b4a6ed47d5d2d7ef2d3d09b1186a8ca225f5e
+Subproject commit 34c0674a251287c94cdd1a112966bcb9010c62e8
