std: Read HOME instead of USER

Apparently one of the linux bots doesn't have the USER variable defined, and
this fix will likely land more quickly than a fix to the bots.
auto merge of #14775 : alexcrichton/rust/fix-master, r=brson

Apparently one of the linux bots doesn't have the USER variable defined, and
this fix will likely land more quickly than a fix to the bots.