[2.x] fix: Fix updateSbtClassifiers using wrong Scala version for cross-built plugins (#8495)

**Problem**

When cross-building sbt plugins with explicit `scalaVersion` set in `build.sbt`, the `updateSbtClassifiers` task fails because it uses the launcher's Scala version instead of the appropriate Scala version for the target sbt version.

For example, with this configuration:
```scala
lazy val root = (project in file("."))
  .enablePlugins(SbtPlugin)
  .settings(
    scalaVersion := "2.10.7",  // Explicit for IDE support
    crossSbtVersions := Seq("0.13.17", "1.0.0"),
  )
```

Running sbt "show updateSbtClassifiers" would fail with:
Error downloading org.scala-sbt:scripted-plugin_2.12:0.13.17

It should look for scripted-plugin_2.10:0.13.17 since sbt 0.13.x uses Scala 2.10.

**Solution**

Modified sbtClassifiersTasks in Defaults.scala to:
1. Check if the project is an sbt plugin (sbtPlugin.value)
2. For plugins, derive the Scala version from pluginCrossBuild / sbtBinaryVersion using PluginCross.scalaVersionFromSbtBinaryVersion
3. For non-plugins, continue using the launcher's Scala version (original behavior)

main/src/main/scala/sbt/Defaults.scala

@@ -3550,7 +3550,13 @@ object Classpaths {
           classifiersModule := Def.uncached(classifiersModuleTask.value),
           // Redefine scalaVersion and scalaBinaryVersion specifically for the dependency graph used for updateSbtClassifiers task.
           // to fix https://github.com/sbt/sbt/issues/2686
-          scalaVersion := appConfiguration.value.provider.scalaProvider.version,
+          // For sbt plugins, use the Scala version corresponding to the sbt binary version being targeted.
+          // to fix https://github.com/sbt/sbt/issues/8026
+          scalaVersion := {
+            val isPlugin = sbtPlugin.value
+            if (isPlugin) (pluginCrossBuild / scalaVersion).value
+            else appConfiguration.value.provider.scalaProvider.version
+          },
           scalaBinaryVersion := binaryScalaVersion(scalaVersion.value),
           scalaEarlyVersion := CrossVersion.earlyScalaVersion(scalaVersion.value),
           scalaOrganization := ScalaArtifacts.Organization,

sbt-app/src/sbt-test/dependency-management/i8026-update-sbt-classifiers-cross-plugin/build.sbt

@@ -0,0 +1,18 @@
+// Test for https://github.com/sbt/sbt/issues/8026
+// When building sbt plugins with explicit scalaVersion set,
+// updateSbtClassifiers should use the correct Scala version for the sbt version.
+
+lazy val root = (project in file("."))
+  .enablePlugins(SbtPlugin)
+  .settings(
+    name := "test-sbt-cross-build",
+
+    // Explicitly set scala version - this is what caused the issue in #8026
+    // When scalaVersion is explicitly set, updateSbtClassifiers was using the
+    // launcher's Scala version instead of the plugin's Scala version.
+    // Before the fix, this would fail with:
+    //   Error downloading org.scala-sbt:scripted-plugin_2.12:0.13.17
+    // because it used the launcher's Scala version (2.12) instead of the
+    // plugin's target Scala version derived from sbt binary version.
+    scalaVersion := "2.12.21",
+  )

sbt-app/src/sbt-test/dependency-management/i8026-update-sbt-classifiers-cross-plugin/project/build.sbt

@@ -0,0 +1 @@
+ThisBuild / pluginCrossBuild / sbtVersion := "2.0.0"

sbt-app/src/sbt-test/dependency-management/i8026-update-sbt-classifiers-cross-plugin/test

@@ -0,0 +1 @@
+> updateSbtClassifiers