This has been replaced by `//build:...` for a long time now.
Removal of the old build tag was automated with:
for i in $(git grep -l '^// +build' | grep -v -e '^vendor/'); do if ! grep -q '^// Code generated' "$i"; then sed -i -e '/^\/\/ +build/d' "$i"; fi; done
Historically it took an exec argument so you could pass a FakeExec to
mock its behavior in unit tests, but it has a fake implementation now
that is much more useful for unit tests than trying to use the real
implementation with a fake exec. (The unit tests still use fake execs,
but they don't need to use a public constructor.) So remove the exec
args from the public constructors.
