lg2: Reset format string for multiple uses

The stderr output format string pointer was incremented during
processing, and never reset, resulting in empty output to cerr on all
log messages beyond the first. This resets the pointer on each call.

Tested: Issued multiple lg2::info calls and observed them all on stderr.

Signed-off-by: Jonathan Doman <jonathan.doman@intel.com>
Change-Id: I1a2b3eaa23604b5260ee61c3167f6956018f034d
diff --git a/lib/lg2_logger.cpp b/lib/lg2_logger.cpp
index bd1e89d..8d9c034 100644
--- a/lib/lg2_logger.cpp
+++ b/lib/lg2_logger.cpp
@@ -134,7 +134,7 @@
 static void cerr_extra_output(level l, const lg2::source_location& s,
                               const std::string& m)
 {
-    static const char* format = []() {
+    static const char* const defaultFormat = []() {
         const char* f = getenv("LG2_FORMAT");
         if (nullptr == f)
         {
@@ -143,6 +143,8 @@
         return f;
     }();
 
+    const char* format = defaultFormat;
+
     while (*format)
     {
         if (*format != '%')