Use shell builtin `command -v` instead non-standard `which`

Reference:
- https://github.com/koalaman/shellcheck/wiki/SC2230

bin/background_jobs_sk

@@ -36,7 +36,7 @@ start_silent()
 start_sidekiq()
 {
   cmd="exec"
-  chpst=$(which chpst)
+  chpst=$(command -v chpst)
 
   if [ -n "$chpst" ]; then
     cmd="${cmd} ${chpst} -P"

bin/background_jobs_sk_cluster

@@ -43,7 +43,7 @@ restart()
 start_sidekiq()
 {
   cmd="exec"
-  chpst=$(which chpst)
+  chpst=$(command -v chpst)
 
   if [ -n "$chpst" ]; then
     cmd="${cmd} ${chpst} -P"

doc/install/installation.md

@@ -142,7 +142,7 @@ Starting with GitLab 12.0, Git is required to be compiled with `libpcre2`.
 Find out if that's the case:
 
 ```shell
-ldd $(which git) | grep pcre2
+ldd $(command -v git) | grep pcre2
 ```
 
 The output should contain `libpcre2-8.so.0`.

qa/qa/git/repository.rb

@@ -111,7 +111,7 @@ module QA
       end
 
       def commit_with_gpg(message)
-        run(%Q{git config user.signingkey #{@gpg_key_id} && git config gpg.program $(which gpg) && git commit -S -m "#{message}"}).to_s
+        run(%Q{git config user.signingkey #{@gpg_key_id} && git config gpg.program $(command -v gpg) && git commit -S -m "#{message}"}).to_s
       end
 
       def push_changes(branch = 'master')