go: crypto/x509: TestSystemVerify consistently failing

#!watchflakes
post <- goos == "windows" && pkg == "crypto/x509" && test == "TestSystemVerify"

https://build.golang.org/log/97b5cac923dc9079392efd9637b9fffa8b9f7592:

--- FAIL: TestSystemVerify (1.59s)
    --- FAIL: TestSystemVerify/Valid (0.10s)
        verify_test.go:479: unexpected error: x509: certificate signed by unknown authority
    --- FAIL: TestSystemVerify/MixedCase (0.00s)
        verify_test.go:479: unexpected error: x509: certificate signed by unknown authority
    --- FAIL: TestSystemVerify/HostnameMismatch (0.00s)
        verify_test.go:378: error was not a HostnameError: x509: certificate signed by unknown authority
    --- FAIL: TestSystemVerify/IPMissing (0.00s)
        verify_test.go:378: error was not a HostnameError: x509: certificate signed by unknown authority
    --- FAIL: TestSystemVerify/Expired (0.00s)
        verify_test.go:388: error was not Expired: x509: certificate signed by unknown authority
    --- FAIL: TestSystemVerify/RootInIntermediates (0.00s)
        verify_test.go:479: unexpected error: x509: certificate signed by unknown authority
    --- FAIL: TestSystemVerify/SHA-384 (0.02s)
        verify_test.go:514: Got 0 matches instead of 1 for expected chain [api.moip.com.br COMODO RSA Extended Validation Secure Server CA COMODO RSA Certification Authority AddTrust External CA Root]
FAIL
FAIL	crypto/x509	2.845s

(attn @golang/security @dmitshur)