Revert "Temporarily disable linux runners on github actions."

This reverts commit 7fbf0da.

Author: arobenko

.github/workflows/actions_build.yml

@@ -9,7 +9,6 @@ env:
 jobs:
 
   build_gcc_old_ubuntu_20_04:
-    if: false
     runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
@@ -69,7 +68,6 @@ jobs:
         VERBOSE: 1     
 
   build_gcc_ubuntu_20_04:
-    if: false
     runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
@@ -122,7 +120,6 @@ jobs:
         VERBOSE: 1 
 
   build_gcc_ubuntu_22_04:
-    if: false
     runs-on: ubuntu-22.04
     strategy:
       fail-fast: false
@@ -179,7 +176,6 @@ jobs:
 
 
   build_clang_old_ubuntu_20_04:
-    if: false
     runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
@@ -244,7 +240,6 @@ jobs:
         VERBOSE: 1 
 
   build_clang_ubuntu_20_04:
-    if: false
     runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
@@ -302,7 +297,6 @@ jobs:
         VERBOSE: 1 
 
   build_clang_ubuntu_22_04:
-    if: false
     runs-on: ubuntu-22.04
     strategy:
       fail-fast: false