feat: added IsNightly to OSS template variables

This should make it easier to use the same config file with both oss and
pro versions when only the nightly feature is used, e.g. in #3501.

Currently, that would fail because `IsNightly` is not a template field,
so you won't really be able to do it without keeping a second
`.goreleaser.yml` file. With this change, `IsNightly` always eval to
false on OSS, and keeps working as before on Pro.

internal/tmpl/tmpl.go

@@ -51,6 +51,7 @@ const (
 	patch           = "Patch"
 	prerelease      = "Prerelease"
 	isSnapshot      = "IsSnapshot"
+	isNightly       = "IsNightly"
 	isDraft         = "IsDraft"
 	env             = "Env"
 	date            = "Date"
@@ -109,6 +110,7 @@ func New(ctx *context.Context) *Template {
 		patch:           ctx.Semver.Patch,
 		prerelease:      ctx.Semver.Prerelease,
 		isSnapshot:      ctx.Snapshot,
+		isNightly:       false,
 		isDraft:         ctx.Config.Release.Draft,
 		releaseNotes:    ctx.ReleaseNotes,
 		releaseURL:      ctx.ReleaseURL,

internal/tmpl/tmpl_test.go

@@ -88,6 +88,7 @@ func TestWithArtifact(t *testing.T) {
 		"2023-03-09T02:06:02Z":             `{{ .Date }}`,
 		"1678327562":                       `{{ .Timestamp }}`,
 		"snapshot true":                    `snapshot {{.IsSnapshot}}`,
+		"nightly false":                    `nightly {{.IsNightly}}`,
 		"draft true":                       `draft {{.IsDraft}}`,
 		"dirty true":                       `dirty {{.IsGitDirty}}`,