Remove unnecessary jq validation of rust-version

`cargo metadata` already validates the version, checking that it's
a string satisfying one of a small number of recognized formats. So
the only validation `jq` needs to do is to make sure it is really
only appending `.0` to the end if it is of the form `X.Y` for
nonempty numeric `X` and `Y` (with no other characters anywhere).

This shortens the `jq` query considerably, removing the unnecessary
validation and also using regular expression replacement to replace
the empty string that follows the a *full* `X.Y` line with `.0`.

Author: EliahKagan

justfile

@@ -242,16 +242,7 @@ check-size:
     etc/check-package-size.sh
 
 # Report the Minimum Supported Rust Version (the `rust-version` of `gix`) in X.Y.Z form
-msrv: (get-metadata '''
-    (.packages[] | select(.name == "gix") | .rust_version | tostring) as $v |
-        if ($v | test("^[0-9]+\\.[0-9]+\\.[0-9]+$")) then
-            $v
-        elif ($v | test("^[0-9]+\\.[0-9]+$")) then
-            $v + ".0"
-        else
-            error("Unrecognized rust-version format: " + $v)
-        end
-''')
+msrv: (get-metadata '.packages[] | select(.name == "gix") | .rust_version | sub("^\\d+\\.\\d+\\K$"; ".0")')
 
 # Regenerate the MSRV badge SVG
 msrv-badge: