Configure structopt to report the right version

By default, structopt reports the value provided by
the env var CARGO_PKG_VERSION, feeded by Cargo when reading
Cargo.toml. However for Garage we use a versioning based on git,
so we often report a version that is behind the real version.
In this commit, we create garage_util::version::garage() that
reports the right version and configure all structopt subcommands
to call this function instead of using the env var.

nix/compile.nix

@@ -129,16 +129,7 @@ let
 
     (pkgs.rustBuilder.rustLib.makeOverride {
       name = "garage_rpc";
-      overrideAttrs = drv:
-        (if git_version != null then {
-          /* [3] */ preConfigure = ''
-            ${drv.preConfigure or ""}
-            export GIT_VERSION="${git_version}"
-          '';
-        } else {})
-        // {
-          /* [1] */ setBuildEnv = (buildEnv drv);
-        };
+      overrideAttrs = drv: { /* [1] */ setBuildEnv = (buildEnv drv); };
       overrideArgs = old: {
         /* [4] */ features = if release then [ "kubernetes-discovery" ] else [];
       };
@@ -151,7 +142,15 @@ let
 
     (pkgs.rustBuilder.rustLib.makeOverride {
       name = "garage_util";
-      overrideAttrs = drv: { /* [1] */ setBuildEnv = (buildEnv drv); };
+      overrideAttrs = drv:
+        (if git_version != null then {
+          /* [3] */ preConfigure = ''
+            ${drv.preConfigure or ""}
+            export GIT_VERSION="${git_version}"
+          '';
+        } else {})
+        // 
+        { /* [1] */ setBuildEnv = (buildEnv drv); };
     })
 
     (pkgs.rustBuilder.rustLib.makeOverride {