x/benchmarks/sweet: change dir when building

[Lslightly]

change dir to cfg.GoRoot to avoid installing cmd/compile cmd/link in pkg/mod specified by benchmarks' go.mod.

Fixes golang/go#72920

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Message from Gopher Robot:

Patch Set 1:

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[Lslightly]

For go version >= version written in go.mod, currently 1.23.0, no problems will occur. For go < 1.23.0, the go.mod will download go whose version >= 1.23.0 automatically as https://go.dev/doc/toolchain says.

For example, when using the go command bundled with Go 1.21.3 in a main module that says go 1.21.9, the go command finds and runs Go 1.21.9 instead.