[release-branch.go1.3] runtime: fix GOTRACEBACK reading on Windows, Plan 9

Only Unix was resetting the traceback_cache variable
after initializing the environment. Reset it on all systems
by resetting in parsedebugvars.

Fixes #8813.

LGTM=adg, alex.brainman
R=golang-codereviews, adg, alex.brainman
CC=golang-codereviews, iant, r
https://golang.org/cl/152760043