sbt: cache passwordCommand output

This will cache the output of `passwordCommand` per authentication
realm.

Context: the `credentials` key in `sbt` is a `TaskKey[Seq[Credentials]]`.
In `sbt`, tasks are evaluated on-demand and their output is not cached.
This particular key is referenced by all submodules in a project. When
the command is relatively expensive (e.g.: `pass show foo`), this
results in several seconds of delay when doing basic things like
`compile` or `test` which makes this unusable without some kind of
caching.

modules/programs/sbt.nix

@@ -8,13 +8,16 @@ let
     addSbtPlugin("${plugin.org}" % "${plugin.artifact}" % "${plugin.version}")
   '';
 
-  renderCredential = cred: ''
-    credentials += Credentials("${cred.realm}", "${cred.host}", "${cred.user}", "${cred.passwordCommand}".!!.trim)
-  '';
+  renderCredential = idx: cred:
+    let symbol = "credential_${toString idx}";
+    in ''
+      lazy val ${symbol} = "${cred.passwordCommand}".!!.trim
+      credentials += Credentials("${cred.realm}", "${cred.host}", "${cred.user}", ${symbol})
+    '';
 
   renderCredentials = creds: ''
     import scala.sys.process._
-    ${concatStrings (map renderCredential creds)}'';
+    ${concatStrings (imap0 renderCredential creds)}'';
 
   renderRepository = value:
     if isString value then ''