Remove snapshot conditional for bwc snapshots (elastic#28657)

The build.snapshot flag used by the main build was being propagated down
into the bwc snapshot builds, which is not correct. The bwc subprojects
are always meant to be snapshot builds, or null if they do not
exist. Marking these builds as non snapshots threw the release off as it
was looking for -SNAPSHOT builds.

Relates elastic#28641

distribution/bwc/build.gradle

@@ -139,7 +139,7 @@ subprojects {
     } else {
       executable new File(checkoutDir, 'gradlew').toString()
     }
-    args ":distribution:deb:assemble", ":distribution:rpm:assemble", ":distribution:zip:assemble", "-Dbuild.snapshot=${System.getProperty('build.snapshot') ?: 'true'}"
+    args ":distribution:deb:assemble", ":distribution:rpm:assemble", ":distribution:zip:assemble", "-Dbuild.snapshot=true"
     final LogLevel logLevel = gradle.startParameter.logLevel
     if ([LogLevel.QUIET, LogLevel.WARN, LogLevel.INFO, LogLevel.DEBUG].contains(logLevel)) {
       args "--${logLevel.name().toLowerCase(Locale.ENGLISH)}"