expvar: avoid conflict with user-defined "GET /" route.

[xpmatteo]

With the new routing style in go 1.22, declaring

http.Handle("GET /", h)

generates a conflict with route "/debug/vars" declared in the expvar
package. You get an error such as:

panic: pattern "GET /" (registered at ...) conflicts with pattern
"/debug/vars" (registered at ...expvar.go:384): GET / matches fewer
methods than /debug/vars, but has a more general path pattern

This patch prevents that error. Adding GET is correct because no other
method makes sense with /debug/vars.

We preserve the traditional behaviour when GODEBUG=httpmuxgo121=1 is
specified.

Fixes #65723

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