cmd/govulncheck/internal/govulncheck: restrict to 1.18 or higher

gopls supports 1.13 and above, but govulncheck only works starting at
1.18.

Change-Id: I6b0b39583a5190f3f3840be62a95e88819d4b6e2
Reviewed-on: https://go-review.googlesource.com/c/vuln/+/406196
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Zvonimir Pavlinovic <zpavlinovic@google.com>
diff --git a/cmd/govulncheck/internal/govulncheck/cache.go b/cmd/govulncheck/internal/govulncheck/cache.go
index 0948837..404c356 100644
--- a/cmd/govulncheck/internal/govulncheck/cache.go
+++ b/cmd/govulncheck/internal/govulncheck/cache.go
@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build go1.18
+// +build go1.18
+
 // Package govulncheck supports the govulncheck command.
 package govulncheck
 
diff --git a/cmd/govulncheck/internal/govulncheck/cache_test.go b/cmd/govulncheck/internal/govulncheck/cache_test.go
index 0917ae8..5a25c78 100644
--- a/cmd/govulncheck/internal/govulncheck/cache_test.go
+++ b/cmd/govulncheck/internal/govulncheck/cache_test.go
@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build go1.18
+// +build go1.18
+
 package govulncheck
 
 import (
diff --git a/cmd/govulncheck/internal/govulncheck/source.go b/cmd/govulncheck/internal/govulncheck/source.go
index 9f91665..752a831 100644
--- a/cmd/govulncheck/internal/govulncheck/source.go
+++ b/cmd/govulncheck/internal/govulncheck/source.go
@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build go1.18
+// +build go1.18
+
 package govulncheck
 
 import (
diff --git a/cmd/govulncheck/internal/govulncheck/util.go b/cmd/govulncheck/internal/govulncheck/util.go
index 6699236..baa2d96 100644
--- a/cmd/govulncheck/internal/govulncheck/util.go
+++ b/cmd/govulncheck/internal/govulncheck/util.go
@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build go1.18
+// +build go1.18
+
 package govulncheck
 
 import (
diff --git a/cmd/govulncheck/internal/govulncheck/util_test.go b/cmd/govulncheck/internal/govulncheck/util_test.go
index 67e7772..3288cd8 100644
--- a/cmd/govulncheck/internal/govulncheck/util_test.go
+++ b/cmd/govulncheck/internal/govulncheck/util_test.go
@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build go1.18
+// +build go1.18
+
 package govulncheck
 
 import (