docs: document the GALLIUM_LOG_FILE env var

docs/envvars.html

@@ -121,6 +121,8 @@ Mesa EGL supports different sets of environment variables.  See the
 <h2>Gallium environment variables</h2>
 
 <ul>
+<li>GALLIUM_LOG_FILE - specifies a file for logging all errors, warnings, etc.
+    rather than stderr.
 <li>GALLIUM_PRINT_OPTIONS - if non-zero, print all the Gallium environment
     variables which are used, and their current values.
 <li>GALLIUM_NOSSE - if non-zero, do not use SSE runtime code generation for