Work around #8256, do not fail the task, just return None
auto merge of #8278 : cmr/rust/workaround, r=brson
std::rt: 2MB stacks again
std::rt: Pull RUST_MIN_STACK from the environment
std: Allow spawners to specify stack size
rustc: Use 4MB stacks. Needed for unoptimized builds apparently.
rustpkg: Disable test_uninstall

Seems to not work
test: xfail a bunch of tests that are incorrectly reading os::args()[1]
xfail task-perf-linked-failure. deadlock
xfail debug-info/option-like-enum

Don't understand why this broke.
Turn on the new runtime