[WIP] ci: Enable RUST_BACKTRACE=1

[kennytm]

1. Enable RUST_BACKTRACE=1 when building (but not testing).
2. Re-enable debug and LLVM assertions for macOS dist builds (they remain disabled in check builds).

WIP — checking if slowdown is within acceptable range (≤2 hours to complete).

[kennytm]

For record:

Image	master	assertions	no debug
x86_64-gnu-llvm-3.9	1h 16m 14s	1h 31m 44s	1h 10m 3s
check i686-apple-darwin	2h 40m 36s	2h 40m 56s	2h 50m 56s
dist i686-apple-darwin	1h 53m 43s	2h 34m 58s	2h 45m 49s
dist x86_64-apple-darwin	2h 20m 07s	--	--
dist-s390x-linux	1h 39m 53s	1h 32m 49s	1h 41m 58s
x86_64-gnu-full-bootstrap	1h 06m 10s	1h 27m 48s	1h 08m 19s

Looks like the impact from debug/LLVM assertions are still huge, and enabling RUST_BACKTRACE=1 for Macs will push them very close to the 3 hr line.

Will try disable RUST_BACKTRACE=1 for macs entirely next.

[kennytm]

Waiting for #45783. Closing for now.