lbutils/file: only override EEXIST if not set

in the linux fast path, we are universally overriding
errno with EEXIST, which pollutes errno in case of
debugging under fault condition. this is inconsistent
with posix and also leah.

Signed-off-by: Leah Rowe <leah@libreboot.org>

util/libreboot-utils/lib/mkhtemp.c

@@ -717,7 +717,8 @@ mkhtemp_tmpfile_linux(int dirfd,
 		goto out;
 	}
 
-	errno = EEXIST;
+	if (!errno)
+		errno = EEXIST;
 err:
 	if (linked)
 		(void) unlinkat(dirfd, fname_copy, 0);