yubikey-touch-detector: add icon

As explained in maximbaz/yubikey-touch-detector#25, YTD doesn't include
an icon but is intended to be paired with one by the user or packager.

Following the example of alpinelinux/aports@1f596c2, this derives an
icon from the assets of Yubico Authenticator.

pkgs/tools/security/yubikey-touch-detector/default.nix

@@ -1,4 +1,4 @@
-{ lib, libnotify, buildGoModule, fetchFromGitHub, pkg-config }:
+{ lib, libnotify, buildGoModule, fetchFromGitHub, fetchurl, pkg-config, iconColor ? "#84bd00" }:
 
 buildGoModule rec {
   pname = "yubikey-touch-detector";
@@ -12,13 +12,31 @@ buildGoModule rec {
   };
   vendorHash = "sha256-OitI9Yp4/mRMrNH4yrWSL785+3mykPkvzarrc6ipOeg=";
 
+  iconSrc = fetchurl {
+    url = "https://github.com/Yubico/yubioath-flutter/raw/yubioath-desktop-5.0.0/images/touch.svg";
+    hash = "sha256-+jC9RKjl1uMBaNqLX5WXN+E4CuOcIEx5IGXWxgxzA/k=";
+  };
+
   nativeBuildInputs = [ pkg-config ];
 
   buildInputs = [ libnotify ];
 
+  postPatch = ''
+    cp $iconSrc yubikey-touch-detector.svg
+    substituteInPlace yubikey-touch-detector.svg \
+      --replace '#284c61' ${lib.escapeShellArg iconColor}
+
+    substituteInPlace notifier/libnotify.go \
+      --replace \
+        'AppIcon: "yubikey-touch-detector"' \
+        "AppIcon: \"$out/share/icons/yubikey-touch-detector.svg\""
+  '';
+
   postInstall = ''
     install -Dm444 -t $out/share/doc/${pname} *.md
 
+    install -Dm444 -t $out/share/icons yubikey-touch-detector.svg
+
     install -Dm444 -t $out/lib/systemd/user *.{service,socket}
 
     substituteInPlace $out/lib/systemd/user/*.service \
@@ -29,7 +47,7 @@ buildGoModule rec {
     description = "A tool to detect when your YubiKey is waiting for a touch (to send notification or display a visual indicator on the screen).";
     homepage = "https://github.com/maximbaz/yubikey-touch-detector";
     maintainers = with maintainers; [ sumnerevans ];
-    license = licenses.isc;
+    license = with licenses; [ bsd2 isc ];
     platforms = platforms.linux;
   };
 }