x/build: delete IN_KUBERNETES env

[bradfitz]

The IN_KUBERNETES environment variable used to mean something, and its meaning morphed over time, and now it's just an overloaded mess that might not even be used.

Delete it, and then replace it as needed with more specific checks.

/cc @bcmills

[gopherbot]

Change https://golang.org/cl/201737 mentions this issue: syscall: remove use of IN_KUBERNETES in test, add a TODO to improve further

[gopherbot]

Change https://golang.org/cl/201739 mentions this issue: all: remove use old, overloaded of IN_KUBERNETES

[gopherbot]

Change https://golang.org/cl/202021 mentions this issue: env/crosscompile: update the three cross compile envs from Jessie to Buster