Fix CI failures by adding apt-get update to Github Actions

[embg]

Following advice from here and here, add sudo apt-get update to hopefully avoid this CI error:

E: Failed to fetch http://azure.archive.ubuntu.com/ubuntu/pool/main/g/glibc/libc6-dev-i386_2.31-0ubuntu9.2_amd64.deb  404  Not Found [IP: 52.250.76.244 80]
E: Failed to fetch http://azure.archive.ubuntu.com/ubuntu/pool/main/g/glibc/libc6-x32_2.31-0ubuntu9.2_amd64.deb  404  Not Found [IP: 52.250.76.244 80]
E: Failed to fetch http://azure.archive.ubuntu.com/ubuntu/pool/main/g/glibc/libc6-dev-x32_2.31-0ubuntu9.2_amd64.deb  404  Not Found [IP: 52.250.76.244 80]
E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?
make[1]: *** [Makefile:337: apt-install] Error 100
make: *** [Makefile:355: libc6install] Error 2
make[1]: Leaving directory '/home/runner/work/zstd/zstd'
Error: Process completed with exit code 2.

The first time I ran CI on this commit (on a PR in embg/zstd) I got a weird failure on a different job, but the second time everything passed. I propose we land this for now (if it passes CI on facebook/zstd), and if the issue persists there are some other things in the first link above that we can try.

[embg]

Well looks like Travis is failing... will investigate tomorrow.

Edit: this link should fix the issue, will test tomorrow. Expect some spam from this PR since I can't test this on embg/zstd (Travis CI doesn't give free credits for public repos anymore).

(note to self: Travis debug instructions here)

Edit: also used this link to fix Travis (there were multiple keys that had to be updated).

[embg]

Fixed most of the Travis issues, but there is one that apparently can only be fixed by moving away from Trusty. So there are two options:

• Move away from Trusty
• Move the "apt-get update" out of the Makefile and have it be specific to Github Actions only (i.e. have Github Actions run "apt-get update" before "make arminstall"). This is a hack that would allow us to continue using Trusty, but should only be a temporary solution since we should really always run "apt-get update" before "apt-get install".

Here is the failing Travis job: https://app.travis-ci.com/github/facebook/zstd/jobs/561969471

[Cyan4973]

The failing travis CI test is a qemu ARM 32-bit test.
This is the kind of test we should no longer need on Travis, and that we should replace with an equivalent test on Github Action.
The reason it's attached to Trusty is likely because we couldn't find a correct combination of flags to correctly compile zstd on this platform, resulting in something working fine implicitly on Trusty but failing on later platforms due to some changes of default, or something like that.

Bottom line : we would be better off removing that test from travis CI, and replace it with an equivalent test on Github Action, that would not need to be run on Trusty. What matters is qemu, and ARM 32-bit.

Oh, and btw, it seems we already have such a test :
https://github.com/facebook/zstd/runs/5410450999?check_suite_focus=true

Just check it's "equivalent enough" to remove the Trusty one from travis CI.

[embg]

Just check it's "equivalent enough" to remove the Trusty one from travis CI.

Sounds good, so for the long term solution I'll get rid of the Trusty test as part of #3011 (sometime this half). Will land the hack fix for now to unbreak CI, and replace with permanent fix once #3011 is closed.

[felixhandte]

Ship it.