[interpreter] do not use basic type as context key
[interpreter] do not use basic type as context key

This is to fix following presubmit error

** Presubmit Warnings **
pre-commit-go: lint, pre-commit (8.24s) failed
golint failed:
starlark/interpreter/interpreter_test.go:188:9: should not use basic type int as key in context.WithValue
pcg: checks failed in 8.06s

Change-Id: I6de78d507847d3599343a9206f2f887a93d9c3c0
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1635053
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Yoshisato Yanagisawa <5d95c5c75e3d691d9c77888fff4498e04f8b7157@google.com>
Reviewed-by: Fumitoshi Ukai <0e88e065dd7f7d7f11da28b068df91c65342563c@chromium.org>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go