Fix flaky test by using slice for stable iteration

Created by: mrnugget

I don't know what changed but before this fix I could pretty reliably get TestBestWorkspaceCreator to fail by running:

go test -run TestBestWorkspaceCreator/different_user -count=10 ./internal/campaigns

It would fail with

--- FAIL: TestBestWorkspaceCreator (0.01s)
    --- FAIL: TestBestWorkspaceCreator/different_user (0.01s)
        expect.go:64: unexpected command: 1 error occurred:
                * unexpected argument at position 4: have="bar" want="foo"
            
        expect.go:108: unexpected number of commands executed: have=1 want=4
FAIL
FAIL    github.com/sourcegraph/src-cli/internal/campaigns       0.536s
FAIL

With the fix here I can't get it to fail anymore, so I'm 99.9% sure that the error was due to range over map not being stable and sometimes setting up the wrong assertions which would then fail.