Allow terminal to be launched with a clean environment on Mac and Linux

[Tyriar]

Fixes #70248

---

Example on Linux, before:

$ env
...
SHLVL=3
...
PATH=/home/daniel/.npm-packages/bin:/home/daniel/tools/node-v10.15.1-linux-x64/bin:/snap/bin:/home/daniel/.npm-packages/bin:/home/daniel/tools/node-v10.15.1-linux-x64/bin:/snap/bin:/sbin:/bin:/usr/bin:/usr/local/bin:/snap/bin

After:

$ env
...
SHLVL=1
...
PATH=/home/daniel/.npm-packages/bin:/home/daniel/tools/node-v10.15.1-linux-x64/bin:/snap/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin