client/logmon: acquire executable in init block

This PR causes the logmon task runner to acquire the binary of the
Nomad executable in an 'init' block, so as to almost certainly get
the name while the nomad file still exists.

This is an attempt at fixing the case where a deleted Nomad file
(e.g. during upgrade) may be getting renamed with a mysterious
suffix first.

If this doesn't work, as a last resort we can literally just trim
the mystery string.

Fixes: #14079

.changelog/14297.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+client/logmon: fixed a bug where logmon cannot find nomad executable
+```

client/logmon/plugin.go

@@ -5,22 +5,27 @@ import (
 	"os"
 	"os/exec"
 
-	hclog "github.com/hashicorp/go-hclog"
-	plugin "github.com/hashicorp/go-plugin"
+	"github.com/hashicorp/go-hclog"
+	"github.com/hashicorp/go-plugin"
 	"github.com/hashicorp/nomad/client/logmon/proto"
 	"github.com/hashicorp/nomad/plugins/base"
 	"google.golang.org/grpc"
 )
 
+var bin = getBin()
+
+func getBin() string {
+	b, err := os.Executable()
+	if err != nil {
+		panic(err)
+	}
+	return b
+}
+
 // LaunchLogMon launches a new logmon or reattaches to an existing one.
 // TODO: Integrate with base plugin loader
 func LaunchLogMon(logger hclog.Logger, reattachConfig *plugin.ReattachConfig) (LogMon, *plugin.Client, error) {
 	logger = logger.Named("logmon")
-	bin, err := os.Executable()
-	if err != nil {
-		return nil, nil, err
-	}
-
 	conf := &plugin.ClientConfig{
 		HandshakeConfig: base.Handshake,
 		Plugins: map[string]plugin.Plugin{