hydra-proxy: fix logo url in error pages

Author: mweinelt

build/nginx-error-pages/403.html

@@ -42,7 +42,7 @@
         <p class="lead">
           <a href="https://nixos.org/nixos">
             <img
-              src="https://nixos.org/logo/nixos-hires.png"
+              src="https://raw.githubusercontent.com/NixOS/nixos-artwork/master/logo/nixos.svg"
               width="500px"
               alt="logo"
             />

build/nginx-error-pages/502.html

@@ -37,7 +37,7 @@
         <p class="lead">
           <a href="https://nixos.org/nixos">
             <img
-              src="https://nixos.org/logo/nixos-hires.png"
+              src="https://raw.githubusercontent.com/NixOS/nixos-artwork/master/logo/nixos.svg"
               width="500px"
               alt="logo"
             />