meson: don't fail if latest tag's commit is signed

meson: don't fail if latest tag's commit is signed

Today this is v248 with 938bdfc0fa737d86eb3ecc70506e11e5f740e0dc, which,
if you don't know about the github webflow key fails to configure with
  meson.build:724:8: ERROR:  String "gpg: Signature made Tue 30 Mar 2021 22:59:02 CEST\ngpg:                using RSA key 4AEE18F83AFDEB23\ngpg: Can't check signature: No public key\n1617137942\n" cannot be converted to int
or, if you do, with
  meson.build:724:8: ERROR:  String 'gpg: Signature made Tue 30 Mar 2021 22:59:02 CEST\ngpg:                using RSA key 4AEE18F83AFDEB23\ngpg: Good signature from "GitHub (web-flow commit signing) <noreply@github.com>" [unknown]\ngpg: WARNING: This key is not certified with a trusted signature!\ngpg:          There is no indication that the signature belongs to the owner.\nPrimary key fingerprint: 5DE3 E050 9C47 EA3C F04A  42D3 4AEE 18F8 3AFD EB23\n1617137942\n' cannot be converted to int