Support "log" property in MCP executor

src/main/java/net/fabricmc/loom/configuration/providers/forge/mcpconfig/ConfigValue.java

@@ -39,6 +39,10 @@ public sealed interface ConfigValue {
 	String OUTPUT = "output";
 	String PREVIOUS_OUTPUT_SUFFIX = "Output";
 	String SRG_MAPPINGS_NAME = "mappings";
+	/**
+	 * A special config value that is the path to a log file if absent.
+	 */
+	String LOG = "log";
 
 	<R> R fold(Function<? super Constant, ? extends R> constant, Function<? super Variable, ? extends R> variable);
 

src/main/java/net/fabricmc/loom/configuration/providers/forge/mcpconfig/McpExecutor.java

@@ -102,6 +102,8 @@ public final class McpExecutor {
 				return LoomGradleExtension.get(project).getSrgProvider().getSrg().toAbsolutePath().toString();
 			} else if (extraConfig.containsKey(name)) {
 				return extraConfig.get(name);
+			} else if (name.equals(ConfigValue.LOG)) {
+				return cache.resolve("log.log").toAbsolutePath().toString();
 			}
 
 			throw new IllegalArgumentException("Unknown MCP config variable: " + name);