Use recommended install-php-extensions download link

[pokgak]

Based on the following warning in the build log that I got while using one of the actions from php-actions:

#############################################################################################################
#                                                                                                           #
#                                            W A R N I N G ! ! !                                            #
#                                                                                                           #
# You are using an unsupported method to get install-php-extensions!                                        #
#                                                                                                           #
# Please update the way you fetch it. Read the instrictions at                                              #
# https://github.com/mlocati/docker-php-extension-installer#usage                                           #
#                                                                                                           #
# For example, if you get this script by fetching                                                           #
# https://raw.githubusercontent.com/mlocati/docker-php-extension-installer/master/install-php-extensions    #
# replace it with                                                                                           #
# https://github.com/mlocati/docker-php-extension-installer/releases/latest/download/install-php-extensions #
#                                                                                                           #
# Sleeping for a while so you get bored of this and act ;)                                                  #
#                                                                                                           #
#############################################################################################################

[g105b]

Thanks for catching this! I wonder how long it's been there without me noticing...