Fix FGP's generateLockfiles task always executing its action at configuration time (#162220)

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This PR attempts to fix #110559

This PR supersedes #147837

Original attempt at fixing this by @GaryQian: #112723

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we expect every widget to implement].
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- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
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Fixes] where supported.
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Author: bartekpacia

packages/flutter_tools/gradle/src/main/groovy/flutter.groovy

@@ -186,13 +186,15 @@ class FlutterPlugin implements Plugin<Project> {
         Project rootProject = project.rootProject
         if (isFlutterAppProject()) {
             rootProject.tasks.register("generateLockfiles") {
-                rootProject.subprojects.each { subproject ->
-                    String gradlew = (OperatingSystem.current().isWindows()) ?
-                        "${rootProject.projectDir}/gradlew.bat" : "${rootProject.projectDir}/gradlew"
-                    rootProject.exec {
-                        workingDir(rootProject.projectDir)
-                        executable(gradlew)
-                        args(":${subproject.name}:dependencies", "--write-locks")
+                doLast {
+                    rootProject.subprojects.each { subproject ->
+                        String gradlew = (OperatingSystem.current().isWindows()) ?
+                            "${rootProject.projectDir}/gradlew.bat" : "${rootProject.projectDir}/gradlew"
+                        rootProject.exec {
+                            workingDir(rootProject.projectDir)
+                            executable(gradlew)
+                            args(":${subproject.name}:dependencies", "--write-locks")
+                        }
                     }
                 }
             }