fixes for linter errors about 'which'

For example:

    in utils/deb/deb-ci.sh line 27: 'which git-secret':
    SC2230: which is non-standard. Use builtin 'command -v' instead.

utils/apk/apk-ci.sh

@@ -24,7 +24,8 @@ function integration_tests {
 
   # Testing the installation:
   apk info | grep "git-secret"
-  which "git-secret"
+  # lint says to use 'command -v' and not 'which'
+  command -v "git-secret"
 
   # Test the manuals:
   man --where "git-secret" # .7

utils/deb/deb-ci.sh

@@ -24,7 +24,8 @@ function integration_tests {
 
   # Testing the installation:
   dpkg --get-selections | grep "git-secret"
-  which "git-secret"
+  # lint says to use 'command -v' and not 'which'
+  command -v "git-secret"
 
   # Test the manuals:
   man --where "git-secret" # .7

utils/make/make-ci.sh

@@ -18,7 +18,9 @@ function integration_tests {
   make install
 
   # Testing the installation:
-  which "git-secret"
+  # 'command -v' is like 'which'
+  command -v "git-secret"
+
 
   # Test the manuals:
   man --where "git-secret" # .7

utils/rpm/rpm-ci.sh

@@ -24,7 +24,8 @@ function integration_tests {
 
   # Testing the installation:
   dnf info "git-secret"
-  which "git-secret"
+  # 'command -v' is like 'which'
+  command -v "git-secret"
 
   # Test the manuals:
   man --where "git-secret" # .7