ci: build and test on linux on CI

The only Linux distributions we were building and testing on are alpine
and debian. This change also adds the linux executor which was unused.

Signed-off-by: Darshan Sen <raisinten@gmail.com>

Author: RaisinTen

.circleci/config.yml

@@ -147,12 +147,12 @@ jobs:
 build_matrix: &build_matrix
   matrix:
     parameters:
-      os: [ alpine, debian, macos, "win/default" ]
+      os: [ alpine, debian, linux, macos, "win/default" ]
 
 test_matrix: &test_matrix
   matrix:
     parameters:
-      os: [ alpine, debian, macos ]
+      os: [ alpine, debian, linux, macos ]
 
 workflows:
   postject: