cmd/govulncheck: update test file for main module vulnerabilities

As of https://go-review.git.corp.google.com/c/go/+/596035, go command
adds a version for a Go binary, not always defaulting to devel. That
causes a devel test to fail at go tip builders. This CL adds an explicit
binary with devel version rather than the test building its own binary.
Once the new Go version with the above fix is released, we'll add
another test.

Change-Id: I409d18c85a0fad9b424771bd330067ac987d4830
Reviewed-on: https://go-review.googlesource.com/c/vuln/+/605855
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Cottrell <iancottrell@google.com>

cmd/govulncheck/testdata/main/testfiles/binary/binary_call_text.ct

@@ -1,6 +1,6 @@
 #####
 # Test no vulnerabilities in main module with devel version.
-$ govulncheck -mode=binary ${main_vuln_binary}
+$ govulncheck -mode=binary ${moddir}/vuln/vuln_main_devel
 No vulnerabilities found.