chore: Switch from mach to mach2

[larseggert]

mach is unmaintained, and this is one of the last two packages in Gecko depending on it:

Crate:     mach
Version:   0.3.2
Warning:   unmaintained
Title:     mach is unmaintained
Date:      2020-07-14
ID:        RUSTSEC-2020-0168
URL:       https://rustsec.org/advisories/RUSTSEC-2020-0168
Dependency tree:
mach 0.3.2
├── cubeb-coreaudio 0.1.0
│   └── gkrust-shared 0.1.0
│       ├── gkrust-gtest 0.1.0
│       └── gkrust 0.1.0
└── audio_thread_priority 0.32.0
    ├── gkrust-shared 0.1.0
    ├── audioipc2-server 0.6.0
    │   └── gkrust-shared 0.1.0
    ├── audioipc2-client 0.6.0
    │   └── gkrust-shared 0.1.0
    └── audioipc2 0.6.0
        ├── audioipc2-server 0.6.0
        └── audioipc2-client 0.6.0

[padenot]

Is this better than #34 that I just merged? :-)

[larseggert]

Whoops! No idea what happened here...