use older ubuntu version to build electrs

This is in order to use an older GLIBC version so that the
binary can be used in CI functional tests for Liana wallet.

wizardsardine/liana#1398 (comment)

Author: jp1ac4

.github/workflows/build_electrs.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-24.04, macos-15]
+        os: [ubuntu-20.04, macos-15]
     env:
       ELECTRS_TAG: "v0.10.6"