net,os: use os.LookupEnv instead of syscall.Getenv
The os package already has a function for retrieving an environment
variable with a ok boolean, we don't need to use syscall directly.
Change-Id: Ife873e7245261ec765b628e31a0e7a7bc4dff934
GitHub-Last-Rev:
bc5f30bf56c87a94e0f17dd67a15aa4b2d7e3fd4
GitHub-Pull-Request: golang/go#69700
Reviewed-on: https://go-review.googlesource.com/c/go/+/616342
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>