use CC=gcc-7 CXX=g++-7 for all unix CI builds

[mseth10]

Description

This PR reverts gcc version update (from gcc-7 to gcc-8) for unix CI builds made in afc76b0. gcc8+ is known to cause memory issues #19623 #18501 . We did a RFC #19688 for the same, and as per community feedback reverting the gcc version back to gcc-7 until we fix the root cause.

We need to install gcc-7 explicitly now as ubuntu version has been upgraded by the same commit from 18 to 20. While ubuntu18 came with gcc-7 as default, ubuntu20 comes with gcc-9.

Also, removing apt-get install cmake as pip3 install cmake already exists. Once tensorrt supports ubuntu 20.04, we can drop the 18.04 image and remove the pip3 installation in favor of apt (as apt install on 20.04 provides a recent enough cmake).

[mxnet-bot]

Hey @mseth10 , Thanks for submitting the PR
All tests are already queued to run once. If tests fail, you can trigger one or more tests again with the following commands:

• To trigger all jobs: @mxnet-bot run ci [all]
• To trigger specific jobs: @mxnet-bot run ci [job1, job2]

---

CI supported jobs: [windows-cpu, clang, miscellaneous, edge, unix-cpu, unix-gpu, website, centos-gpu, windows-gpu, centos-cpu, sanity]

---

Note:
Only following 3 categories can trigger CI :PR Author, MXNet Committer, Jenkins Admin.
All CI tests must pass before the PR can be merged.

[leezu]

You need to install gcc-7