Substitute ${PID} variables with file logging

Update DefaultLogbackConfiguration to include OptionHelper.substVars
when setting the encoder pattern so that ${PID} is correctly expanded.

Fixes gh-2119

spring-boot/src/main/java/org/springframework/boot/logging/logback/DefaultLogbackConfiguration.java

@@ -112,7 +112,7 @@ class DefaultLogbackConfiguration {
 			String logFile) {
 		RollingFileAppender<ILoggingEvent> appender = new RollingFileAppender<ILoggingEvent>();
 		PatternLayoutEncoder encoder = new PatternLayoutEncoder();
-		encoder.setPattern(FILE_LOG_PATTERN);
+		encoder.setPattern(OptionHelper.substVars(FILE_LOG_PATTERN, config.getContext()));
 		appender.setEncoder(encoder);
 		config.start(encoder);