replace '-Cinline-threshold' with '-Copt-level=0' in CI

[xxchan]

mozilla/grcov#411

I think this may increase coverage……

[coveralls]

Pull Request Test Coverage Report for Build 165487004

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1197 unchanged lines in 53 files lost coverage.
• Overall coverage increased (+8.1%) to 29.604%

---

Files with Coverage Reduction	New Missed Lines	%
../../../../../usr/share/rust/.cargo/git/checkouts/rcore-fs-7fdf258332f6146d/e17b27b/rcore-fs/src/vfs.rs	1	0.0%
../../../../../usr/share/rust/.cargo/registry/src/github.com-1ecc6299db9ec823/futures-channel-0.3.5/src/oneshot.rs	1	0.0%
../../../../../usr/share/rust/.cargo/registry/src/github.com-1ecc6299db9ec823/smol-0.1.18/src/run.rs	1	95.45%
../../../../../usr/share/rust/.cargo/registry/src/github.com-1ecc6299db9ec823/spin-0.5.2/src/once.rs	1	92.59%
zircon-object/src/object/mod.rs	1	81.97%
zircon-object/src/signal/event.rs	1	0.0%
zircon-object/src/util/elf_loader.rs	1	81.63%
zircon-syscall/src/task.rs	1	0.0%
kernel-hal/src/dummy.rs	2	4.48%
kernel-hal/src/future.rs	2	0.0%

Totals
Change from base Build 164663854:	8.1%
Covered Lines:	3487
Relevant Lines:	11779

---

💛 - Coveralls