GODEBUG started being used for configuring the behavior of the Go
toolchain and standard library, for the sake of smoother transitions
in terms of backwards and forwards compatibility.
See: https://go.dev/doc/godebug
As such, it is not right to have `garble build -tiny` ignore all
GODEBUG settings, because many GODEBUG keys nowadays do not actually
involve debugging what a Go binary is doing.
Moreover, the mechanism we were using broke with Go 1.25.2,
which refactored `func parsedebugvars()` into
`func parseRuntimeDebugVars(godebug string)`,
so our test started breaking as our runtime patching was broken.
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