Fix linux CI (#8310) (#8319)

diff --git a/.github/workflows/main-unix.yml b/.github/workflows/main-unix.yml
index 7ddb3d1..937353f 100644
--- a/.github/workflows/main-unix.yml
+++ b/.github/workflows/main-unix.yml
@@ -37,7 +37,7 @@
       max-parallel: 20
       matrix:
         java: [ 8, 11, 17 ]
-        os: [ ubuntu-latest ]
+        os: [ ubuntu-20.04 ]
         include:
           - java: 8
             os: macos-latest