wal-g: enable support for brotli compression

Brotli support was always on until v0.2.10. It is enabled by default
in the wal-g's 'official' releases and build instructions, so it makes
sense to enable it in nixpkgs too.

wal-g has a Makefile (not used by nixpkgs) which statically links to
brotli v1.0.7 (a C library). Nixpkgs dynamically links to brotli
v1.0.9.

pkgs/tools/backup/wal-g/default.nix

@@ -17,7 +17,10 @@ buildGoModule rec {
 
   subPackages = [ "main/pg" ];
 
-  buildFlagsArray = [ "-ldflags=-s -w -X github.com/wal-g/wal-g/cmd/pg.WalgVersion=${version} -X github.com/wal-g/wal-g/cmd/pg.GitRevision=${src.rev}" ];
+  buildFlagsArray = [
+    "-tags=brotli"
+    "-ldflags=-s -w -X github.com/wal-g/wal-g/cmd/pg.WalgVersion=${version} -X github.com/wal-g/wal-g/cmd/pg.GitRevision=${src.rev}"
+  ];
 
   postInstall = ''
     mv $out/bin/pg $out/bin/wal-g