homepage-dashboard: prefix nixpkgs-specific env var with NIXPKGS_

I thought HOMEPAGE_CACHE_DIR was an upstream env var, and it confused me
a lot. Let's prefix it with NIXPKGS_ to make it clear that it's nixpkgs
specific.

nixos/modules/services/misc/homepage-dashboard.nix

@@ -240,7 +240,7 @@ in
 
         environment = {
           HOMEPAGE_CONFIG_DIR = configDir;
-          HOMEPAGE_CACHE_DIR = "/var/cache/homepage-dashboard";
+          NIXPKGS_HOMEPAGE_CACHE_DIR = "/var/cache/homepage-dashboard";
           PORT = toString cfg.listenPort;
           LOG_TARGETS = lib.mkIf managedConfig "stdout";
         };

pkgs/servers/homepage-dashboard/default.nix

@@ -76,14 +76,14 @@ buildNpmPackage rec {
     # write its prerender cache.
     #
     # This patch ensures that the cache implementation respects the env
-    # variable `HOMEPAGE_CACHE_DIR`, which is set by default in the
+    # variable `NIXPKGS_HOMEPAGE_CACHE_DIR`, which is set by default in the
     # wrapper below.
     (cd "$out" && patch -p1 <${./prerender_cache_path.patch})
 
     makeWrapper $out/share/homepage/server.js $out/bin/homepage \
       --set-default PORT 3000 \
       --set-default HOMEPAGE_CONFIG_DIR /var/lib/homepage-dashboard \
-      --set-default HOMEPAGE_CACHE_DIR /var/cache/homepage-dashboard
+      --set-default NIXPKGS_HOMEPAGE_CACHE_DIR /var/cache/homepage-dashboard
 
     ${if enableLocalIcons then installLocalIcons else ""}
 

pkgs/servers/homepage-dashboard/prerender_cache_path.patch

@@ -12,7 +12,7 @@ index b1b74d8..a46c80b 100644
          this.fs = ctx.fs;
          this.flushToDisk = ctx.flushToDisk;
 -        this.serverDistDir = ctx.serverDistDir;
-+        this.serverDistDir = path.join(process.env.HOMEPAGE_CACHE_DIR, "homepage");
++        this.serverDistDir = path.join(process.env.NIXPKGS_HOMEPAGE_CACHE_DIR, "homepage");
          this.appDir = !!ctx._appDir;
          if (ctx.maxMemoryCacheSize) {
              this.memoryCache = new _lruCache.default({