This CL is a pure refactoring. This removes a global variable ctx,
which is a build.Default.
Before this change, ctx was used to keep build tags and its state
affected go command executions. As the variable is mutable, the
code was not readable.
This changes introduces another global variable buildTags instead,
but this is more consistent with other build flags, and this is
immutable.
Updates golang/go#27234
Change-Id: Id8d0c779de21b249e96febd2f40833cd0c84534f
Reviewed-on: https://go-review.googlesource.com/c/mobile/+/208060
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
d9e324ca8c