Fix mysql_server_vault CI test

[shlomi-noach]

Description

The test is broken due to EXTRA_BIN directory non-existing. We ttempt to download the vault binary onto that directory, which of course fails:

vitess/go/test/endtoend/vault/vault_server.go

Lines 61 to 65 in e4dc872

	vs.execPath = path.Join(os.Getenv("EXTRA_BIN"), vaultExecutableName)
	fileStat, err := os.Stat(vs.execPath)
	if err != nil || fileStat.Size() != vaultDownloadSize {
	log.Warningf("Downloading Vault binary to: %v", vs.execPath)
	err := downloadExecFile(vs.execPath, vaultDownloadSource)

Initial commit: no fix; reproduce.

Related Issue(s)

This bug was there for a while. Exposed by #17354.

Checklist

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[vitess-bot]

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[shlomi-noach]

Irrelevant. Issue was caused by the removal of rippled binary in #17361 and will be addressed there.