Switch build target from main.go to a package.

This changes the way go embeds versions in the binary. Today, Grype
can't determine which k3s version is used in k3s itself because it's built
with the file. Here's what a scan looks like:

```
NAME                   INSTALLED  FIXED-IN  TYPE       VULNERABILITY        SEVERITY
github.com/k3s-io/k3s  (devel)    1.24.17   go-module  GHSA-m4hf-6vgr-75r2  High
```

If you make this switch, the scanner can correctly determine the version instead of (devel).

Signed-off-by: Dan Lorenc <[email protected]>

scripts/build

@@ -134,7 +134,7 @@ if [ ! -x ${INSTALLBIN}/cni ]; then
 fi
 
 echo Building k3s
-CGO_ENABLED=1 "${GO}" build $BLDFLAGS -tags "$TAGS" -gcflags="all=${GCFLAGS}" -ldflags "$VERSIONFLAGS $LDFLAGS $STATIC" -o bin/k3s ./cmd/server/main.go
+CGO_ENABLED=1 "${GO}" build $BLDFLAGS -tags "$TAGS" -gcflags="all=${GCFLAGS}" -ldflags "$VERSIONFLAGS $LDFLAGS $STATIC" -o bin/k3s ./cmd/server
 ln -s k3s ./bin/containerd
 ln -s k3s ./bin/crictl
 ln -s k3s ./bin/ctr