Renaming platform binary to mattermost. (#8801)

Makefile

@@ -56,7 +56,7 @@ GO_LINKER_FLAGS ?= -ldflags \
 # GOOS/GOARCH of the build host, used to determine whether we're cross-compiling or not
 BUILDER_GOOS_GOARCH="$(shell $(GO) env GOOS)_$(shell $(GO) env GOARCH)"
 
-PLATFORM_FILES="./main.go"
+PLATFORM_FILES="./cmd/mattermost/"
 
 # Output paths
 DIST_ROOT=dist