The noninteractive env var had to be added since tzdata would try to ask
for a region.
ADDs changed to COPYs as it is good practice in the docker community.

Also:
- Remove fixed GCC_VER constraint (7.2.0 is not supported anymore)
- Use rust musl make 0.9.9 (patch level update)
- Use openssl 1.0.2u (patch level update)

Since there is clean up at each step, there is not much overhead, but it
with debugging.

Also:
- Add LD=$TARGET-ld
  The HF targets seem to be happier with it.
- Organize env vars and dedupe hardcoded paths

We need to add an CFLAGS env var for armv7-unknown-linux-musleabihf on
docker run to make the compiler happy.

Error was:

```
warning: cc1: error: '-mfloat-abi=hard': selected processor lacks an FPU
```

Can be fixed by using either of them:
export CFLAGS_armv7_unknown_linux_musleabihf='-mfpu=vfpv3-d16'
export CFLAGS_armv7_unknown_linux_musleabihf='-mfpu=neon'

References:
- influxdata/flux#3379 (comment)
- https://wiki.debian.org/ArmHardFloatPort/VfpComparison#FPU

Per documentation I use "vfpv3-d16" for the CI step. In the end the user
this cross compiler tool needs to make the appropriate decision in their
projects.

On GitHub workflows this seems to be unreliable sometimes,
it randomly fails for some targets.