ci: fix Ubuntu version for kernel tests and pahole workflows

Having too new build environment in workflows that build selftests on
the host, but run them in a separate QEMU image can lead to problems
with runtime linker complaining about missing new enough version of
glibc and other dependencies.

Until we update images, fix used Ubuntu version to ubuntu-20.04 to
mitigate.

Suggested-by: Manu Bretelle <chantr4@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

.github/workflows/pahole.yml

@@ -7,7 +7,7 @@ on:
 
 jobs:
   vmtest:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     name: Kernel LATEST + staging pahole
     env:
       STAGING: tmp.master

.github/workflows/test.yml

@@ -19,13 +19,13 @@ jobs:
       matrix:
         include:
           - kernel: 'LATEST'
-            runs_on: ubuntu-latest
+            runs_on: ubuntu-20.04
             arch: 'x86_64'
           - kernel: '5.5.0'
-            runs_on: ubuntu-latest
+            runs_on: ubuntu-20.04
             arch: 'x86_64'
           - kernel: '4.9.0'
-            runs_on: ubuntu-latest
+            runs_on: ubuntu-20.04
             arch: 'x86_64'
           - kernel: 'LATEST'
             runs_on: s390x