docs: add man page to nix

Signed-off-by: jolheiser <john.olheiser@gmail.com>

flake.nix

@@ -30,6 +30,13 @@
 
           doCheck = false;
 
+          nativeBuildInputs = [pkgs.installShellFiles];
+
+          preInstall = ''
+            go run man.go
+            installManPage git-age.1
+          '';
+
           meta = with pkgs.lib; {
             description = "";
             homepage = "https://git.jojodev.com/jolheiser/git-age";

man.go

@@ -0,0 +1,25 @@
+//go:build man
+
+package main
+
+import (
+	"os"
+
+	"go.jolheiser.com/git-age/cmd"
+)
+
+func main() {
+	fi, err := os.Create("git-age.1")
+	if err != nil {
+		panic(err)
+	}
+	defer fi.Close()
+
+	app := cmd.New()
+	man, err := app.ToMan()
+	if err != nil {
+		panic(err)
+	}
+
+	fi.WriteString(man)
+}