Check if the zip utility is installed before any actions

[pzhlkj6612]

Changes Proposed in this Pull Request:

• Check if the zip utility is installed before any actions

Additional Comments and Documentation:

Before:

$ zip
Command 'zip' not found, but can be installed with:
sudo apt install zip

$ ./build.sh -v '1.4.4'
./build.sh: Creating the build directory structure under .../find/.build...
mkdir: created directory '.../find/.build'
mkdir: created directory '.../find/.build/chr'
mkdir: created directory '.../find/.build/moz'
./build.sh: Copying project source files to build directory...
./build.sh: Updating version number in manifest to 1.4.4...
./build.sh: Packaging extension for Chrome...
./build.sh: line 109: zip: command not found
./build.sh: Packaging extension for Firefox...
./build.sh: line 116: zip: command not found

After:

$ zip
Command 'zip' not found, but can be installed with:
sudo apt install zip

$ ./build.sh -v '1.4.4'
Error: missing 'zip' utility.

.