Move to setting version info explicitly in linker flags

We don't have go built-in VCS information in a nix build because the git
repository isn't present. After struggling to build an overlay that
would provide it, I decided this path is easier of just injecting in the
data that we need.

Issue: #5

flake.nix

@@ -21,8 +21,32 @@
 				projPkg = proj.packages.${system}.default;
 
 				package = pkgs.callPackage ./default.nix {
-				proj = projPkg;
-			};
+					ldflags = ldflags;
+					proj = projPkg;
+					version = packageVersion;
+				};
+				# Pull VCS metadata from the flake's own git source.
+				# These fields are available when the flake lives inside a git checkout.
+				gitRevision = self.sourceInfo.rev or "dirty";
+				gitShortRev = self.sourceInfo.shortRev or "dirty";
+				gitDateTime = self.sourceInfo.lastModifiedDate or "unknown";
+
+				# Derive a human-readable version from the git date + short rev.
+				# lastModifiedDate is YYYYMMDDHHMMSS; pull out just YYYY-MM-DD.
+				packageVersion =
+				let
+				y  = builtins.substring 0 4 gitDateTime;
+				m  = builtins.substring 4 2 gitDateTime;
+				d  = builtins.substring 6 2 gitDateTime;
+				in "${y}-${m}-${d}.${gitShortRev}";
+
+				ldflags = [
+				"-s" "-w"
+				"-X main.BuildTime=${gitDateTime}"
+				"-X main.Revision=${gitRevision}"
+				"-X main.Version=${packageVersion}"
+       				];
+
 			in
 			{
 				packages.default = package;