wp-env.json: Pin tt1-blocks dependency to v0.4.3 (#28741)

Gutenberg's end-to-end tests were broken for a while after TT1 Blocks removed their `front-page` template (WordPress/theme-experiments#179) -- which our e2e tests relied on. This was fixed on the Gutenberg side in #28638.

Per [this discussion](#28692 (comment)), we might be able to avoid a similar situation in the future by pinning our TT1 Blocks dependency to a given version.

See #28692 for more background.

.wp-env.json

@@ -3,7 +3,7 @@
 	"plugins": [
 		"."
 	],
-	"themes": [ "WordPress/theme-experiments/tt1-blocks" ],
+	"themes": [ "WordPress/theme-experiments/tt1-blocks#tt1-blocks@0.4.3" ],
 	"env": {
 		"tests": {
 			"mappings": {

packages/env/lib/config/test/config.js

@@ -366,6 +366,7 @@ describe( 'readConfig', () => {
 							'WordPress/gutenberg',
 							'WordPress/gutenberg#master',
 							'WordPress/gutenberg#5.0',
+							'WordPress/theme-experiments/tt1-blocks#tt1-blocks@0.4.3',
 						],
 					} )
 				)
@@ -394,6 +395,16 @@ describe( 'readConfig', () => {
 						path: expect.stringMatching( /^\/.*gutenberg$/ ),
 						basename: 'gutenberg',
 					},
+					{
+						type: 'git',
+						url:
+							'https://github.com/WordPress/theme-experiments.git',
+						ref: 'tt1-blocks@0.4.3',
+						path: expect.stringMatching(
+							/^\/.*theme-experiments\/tt1-blocks$/
+						),
+						basename: 'tt1-blocks',
+					},
 				],
 			};
 			expect( config.env.tests ).toMatchObject( matchObj );