Add spiffxp to OWNERS files in bash-heavy dirs

I'm comfortable approving changes in hack/, I think I still need
to build up a corpus of reviews in build/ and cluster/ before I'm
comfortable asking for those rights

build/OWNERS

@@ -3,6 +3,7 @@ reviewers:
   - ixdy
   - jbeda
   - lavalamp
+  - spiffxp
   - zmerlynn
 approvers:
   - cblecker

cluster/OWNERS

@@ -3,6 +3,7 @@ reviewers:
   - jbeda
   - mikedanese
   - roberthbailey
+  - spiffxp
   - zmerlynn
 approvers:
   - eparis

hack/OWNERS

@@ -10,6 +10,7 @@ reviewers:
   - sttts
   - gmarek
   - vishh
+  - spiffxp
 approvers:
   - cblecker
   - deads2k
@@ -28,3 +29,4 @@ approvers:
   - liggitt
   - soltysh # for sig-cli related stuff
   - dims # for local-up-cluster related files
+  - spiffxp