postmarketos-android-recovery-installer fails shellcheck

FYI @ata2001

Not sure why Travis didn't catch this, but I'm unable to run static_code_analysis after the android recovery installer PR was merged:

$ ./test/static_code_analysis.sh                                                                 
Test with shellcheck: ./test/static_code_analysis.sh                                      
Test with shellcheck: ./aports/main/postmarketos-base/firmwareload.sh                     
Test with shellcheck: ./aports/main/postmarketos-mkinitfs/init.sh.in                      
Test with shellcheck: ./aports/main/postmarketos-mkinitfs/init_functions.sh               
Test with shellcheck: ./aports/main/postmarketos-android-recovery-installer/build_zip.sh  

In build_zip.sh line 1:                      
#!/bin/ash                                   
^-- SC2187: Ash scripts will be checked as Dash. Add '# shellcheck shell=dash' to silence. \