Fix Go dependency install instructions in README

Prior to Go 1.17 the "go get" command was overloaded to install library
dependencies and applications. As of Go 1.17 installing applications
with it results in a warning, and as of Go 1.18 it will no longer be
used to install applications.

This patch updates the readme to use "go install" instead.

TL;DR — "go get" updates module dependencies, "go install" builds
        binaries and sticks them in $GOBIN.

Signed-off-by: Sam Whited <sam@samwhited.com>

README.md

@@ -23,9 +23,9 @@ Other Linux distributions might need additional steps to install all requirement
 
 ```shell
 sudo apt-get install golang build-essential bind9utils python3-pysodium python3-jinja2 python3-netaddr python3-openssl python3-yaml python3-six python3-crypto ansible git
-go get -u git.autistici.org/ale/x509ca
-go get -u git.autistici.org/ale/ed25519gen
-go get git.autistici.org/ai3/go-common/cmd/pwtool
+go install git.autistici.org/ale/x509ca@latest
+go install git.autistici.org/ale/ed25519gen@latest
+go install git.autistici.org/ai3/go-common/cmd/pwtool@latest
 export PATH=$PATH:$HOME/go/bin
 ```