ci: Disable LLVM/debug assertions for asmjs builder

This shaves of 50 minutes of cycle time on Azure and will likely also
save a significant chunk of time on Travis. The assertions here aren't
really buying us much over other builders with assertions already
enabled, so let's disable them for this builder.

cc #61185

Author: alexcrichton

src/ci/docker/asmjs/Dockerfile

@@ -37,3 +37,11 @@ ENV SCRIPT python2.7 ../x.py test --target $TARGETS \
   src/libstd \
   src/liballoc \
   src/libcore
+
+# Debug assertions in rustc are largely covered by other builders, and LLVM
+# assertions cause this builder to slow down by quite a large amount and don't
+# buy us a huge amount over other builders (not sure if we've ever seen an
+# asmjs-specific backend assertion trip), so disable assertions for these
+# tests.
+ENV NO_LLVM_ASSERTIONS=1
+ENV NO_DEBUG_ASSERTIONS=1