all: disable some prompts and welcome page popup in certain web-based editor contexts

[hyangah]

CloudShell IDE: Check CLOUD_SHELL="true"
Github Codespace: Check CODESPACES="true"

cc @suzmue @stamblerre

[gopherbot]

Change https://golang.org/cl/297690 mentions this issue: src/config: suppress welcome page & update check in cloud-based ides

[zanminkian]

It works for me.