Only read the first line for shbang.

I noticed this when I was playing with a shbang program like:

   #!/usr/bin/env busybox sh
   echo hello world

Melange would give a warning like this:

   Error reading shbang from usr/bin/hello-busybox:
      a shbang of only '/usr/bin/env' with multiple arguments
         (6 /usr/bin/env busybox sh echo "hello world")

The new message would be:

   Error reading shbang from usr/bin/hello-busybox:
      a shbang of only '/usr/bin/env' with multiple arguments
         (3 /usr/bin/env busybox sh)

pkg/sca/sca.go

@@ -602,7 +602,8 @@ func getShbang(fp fs.File) (string, error) {
 		return "", nil
 	}
 
-	toks := strings.Fields(string(buf[2 : blen-2]))
+	line1 := strings.Split(string(buf[2:blen-2]), "\n")[0]
+	toks := strings.Fields(line1)
 	bin := toks[0]
 
 	// if #! is '/usr/bin/env foo', then use next arg as the dep