Use distribution version in the detached DistributionResolver

Update logic to use version from distribution. It no longer needs to use
 overridden version in system property as it should be the same as in
 the distribution.

Author: jozala

build-tools-internal/src/main/java/org/elasticsearch/gradle/internal/InternalDistributionDownloadPlugin.java

@@ -108,12 +108,12 @@ private void registerInternalDistributionResolutions(List<DistributionResolution
         //
         // -Dtests.bwc.refspec.main=deadbeef -Dtests.bwc.main.version=9.0.0
         //
-        // The 'test.bwc.main.version' property should map to the version returned by the commit referenced in 'tests.bwc.refspec.main'.
+        // The distribution version set by 'test.bwc.main.version' property should map to the version returned by the commit
+        // referenced in 'tests.bwc.refspec.main'.
         resolutions.add(new DistributionResolution("detached", (project, distribution) -> {
             if (distribution.isDetachedVersion()) {
-                String versionProperty = System.getProperty("tests.bwc.main.version");
                 BwcVersions.UnreleasedVersionInfo unreleasedVersionInfo = new BwcVersions.UnreleasedVersionInfo(
-                    Version.fromString(versionProperty),
+                    Version.fromString(distribution.getVersion()),
                     "main",
                     ":distribution:bwc:main"
                 );