Make all alt builders produce parallel-enabled compilers

We're not quite ready to ship parallel compilers by default, but the alt
builders are not used too much (in theory), so we believe that shipping
a possibly-broken compiler there is not too problematic.

src/ci/run.sh

@@ -55,6 +55,9 @@ if [ "$DEPLOY$DEPLOY_ALT" = "1" ]; then
   if [ "$NO_LLVM_ASSERTIONS" = "1" ]; then
     RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --disable-llvm-assertions"
   elif [ "$DEPLOY_ALT" != "" ]; then
+    if [ "$NO_PARALLEL_COMPILER" = "" ]; then
+      RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --set rust.parallel-compiler"
+    fi
     RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --enable-llvm-assertions"
     RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --set rust.verify-llvm-ir"
   fi