Add explicit conda-build pinning to sysroot 2.28

Author: antonwolfy

conda-recipe/conda_build_config.yaml

@@ -0,0 +1,4 @@
+c_stdlib:                     # [linux]
+    - sysroot                 # [linux]
+c_stdlib_version:             # [linux]
+    - '2.28'                  # [linux]