Set bash shell as preferred for make

[mjgrzybek]

Description

After recent Makefile refactoring, SHELL = bash disappeared.
By default sh is used and it lacks of bashisms like source which is used in e.g. go/test target:

$ make go/test
...                                                                               
source /home/michal/workspace/dynatrace-operator//testbin/setup-envtest.sh; fetch_envtest_tools /home/michal/workspace/dynatrace-operator//testbin; setup_env
test_env /home/michal/workspace/dynatrace-operator//testbin; go test ./... -coverprofile cover.out                                                                         
/bin/sh: 1: source: not found                                                                                                                                                            
/bin/sh: 1: fetch_envtest_tools: not found                                                                                                                                               
/bin/sh: 1: setup_envtest_env: not found                                                                                                                                                 
?       github.com/Dynatrace/dynatrace-operator/src/agproxysecret       [no test files]                                                                                                  
ok      github.com/Dynatrace/dynatrace-operator/src/api/v1alpha1        0.113s  coverage: 7.7% of statements                                                                             
ok      github.com/Dynatrace/dynatrace-operator/src/api/v1beta1 0.048s  coverage: 31.4% of statement

Set SHELL = bash fixes the issue.

How can this be tested?

make with any target works properly
e.g.: make go/test

Checklist

• Unit tests have been updated/added
• PR is labeled accordingly