Skip -d flag for "go get" if go 1.18 or later is run

[rarkins]

Describe the proposed change(s).

As originally proposed in #32506 but not completed, the -d flag is not necessary for go 1.18 or later.

It's important that Renovate to keep the -d flag if an older version of go is run, although that is rare.

The go toolchain directive was introduced in Go 1.21 so it's not possible for it to be a scenario where <1.18 is used.

The only way which Go could be pinned to a lower version is if the user configures constraints.go in Renovate config, so this edge case should be considered.