util/def,cmd/containerboot: add LookupEnv, simplify env parsing (#20277)

Simplifies cmd/containerboot env var parsing. Most of the private helpers did
not earn their abstraction: defaultEnv(name, "") is just os.Getenv(name), and
the rest collapse into cmp.Or and the existing def.Bool. defaultEnv,
defaultEnvs and defaultBool are gone.

Adds def.LookupEnv, the env companion to def.Bool, for the one case that needs
it: TS_KUBE_SECRET, where an explicit "" disables Kubernetes secret storage and
must stay distinct from unset (cmp.Or cannot express that).

Updates #20018

Signed-off-by: Nick Rossi <nrossi0530@gmail.com>

util/def/def_test.go

@@ -4,6 +4,7 @@
 package def_test
 
 import (
+	"os"
 	"strconv"
 	"testing"
 	"time"
@@ -11,6 +12,32 @@ import (
 	"tailscale.com/util/def"
 )
 
+func TestLookupEnv(t *testing.T) {
+	const key = "TS_DEF_TEST_LOOKUPENV"
+	tests := []struct {
+		name  string
+		unset bool
+		value string
+		def   string
+		want  string
+	}{
+		{name: "unset", unset: true, def: "fallback", want: "fallback"},
+		{name: "set", value: "value", def: "fallback", want: "value"},
+		{name: "empty", value: "", def: "fallback", want: ""},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Setenv(key, tt.value)
+			if tt.unset {
+				os.Unsetenv(key)
+			}
+			if got := def.LookupEnv(key, tt.def); got != tt.want {
+				t.Errorf("LookupEnv(%q, %q) = %q; want %q", key, tt.def, got, tt.want)
+			}
+		})
+	}
+}
+
 func TestBool(t *testing.T) {
 	tests := []struct {
 		name string