cmd/go: TestScript/mod_import failure on darwin-amd64-nocgo

darwin-amd64-nocgo (https://build.golang.org/log/b64b26f0f1ea99d0a15dbe847285ee310af7fae9):

go test proxy running at GOPROXY=http://127.0.0.1:57008/mod
go proxy: no archive rsc.io v1.1.0: file does not exist
go proxy: no archive rsc.io v1.5.2: file does not exist
go proxy: no archive rsc.io v1.0.0: file does not exist
go proxy: no archive rsc.io v1.0.0: file does not exist
go proxy: no archive rsc.io v1.0.0: file does not exist
go proxy: no archive rsc.io v1.0.0: file does not exist
go proxy: no archive rsc.io v1.5.1: file does not exist
go proxy: no archive example.com/newcycle v1.0.0: file does not exist
go proxy: no archive rsc.io v1.5.2: file does not exist
--- FAIL: TestScript (0.01s)
    --- FAIL: TestScript/mod_import (0.79s)
        script_test.go:192: 
            # latest rsc.io/quote should be v1.5.2 not v1.5.3-pre1 (0.648s)
            > go list
            [stdout]
            x
            > go list -m all
            [stdout]
            x
            > stdout 'rsc.io/quote v1.5.2'
            FAIL: testdata/script/mod_import.txt:6: no match for `(?m)rsc.io/quote v1.5.2` found in stdout
            
FAIL
FAIL	cmd/go	251.813s

I'm not sure what to make of this, but the file does not exist error suggest perhaps filesystem flakiness. (There is precedent for the filesystem being flaky in #33041.)

CC @jayconrod