Mention nix develop in readme

Author: neko-kai

README.md

@@ -109,6 +109,12 @@ Likewise with Scala 3:
 
 In Intellij, you may also set Scala version by changing the option `sbt -> sbt settings -> Open cross-compiled projects Scala 3 / Scala 2 projects as:`
 
+Provided `flake.nix` can be used to set up the external dependencies necessary to build the project, such as sbt, JDK, coursier, etc:
+
+```bash
+nix develop
+```
+
 <!--- docs:start --->
 
 # Talks

devops/.env.sh

@@ -9,7 +9,8 @@ export SCALA212=$(cat project/Deps.sc | grep 'val scala212 ' |  sed -r 's/.*\"(.
 export SCALA213=$(cat project/Deps.sc | grep 'val scala213 ' |  sed -r 's/.*\"(.*)\".**/\1/')
 export SCALA3=$(cat project/Deps.sc | grep 'val scala300 ' |  sed -r 's/.*\"(.*)\".**/\1/')
 
-[[ -z "$SCALA_VERSION" ]] && echo "Missing SCALA_VERSION" && exit 1
+[[ -z "${SCALA_VERSION:-}" ]] && echo "Missing SCALA_VERSION. Define SCALA_VERSION" && exit 1
+[[ -z "${JAVA_VERSION:-}" ]] && echo "Missing JAVA_VERSION. Define JAVA_VERSION" && exit 1
 
 case $SCALA_VERSION in
   2.11) SCALA_VERSION="$SCALA211" ;;

flake.nix

@@ -37,9 +37,10 @@
             export JDK21=${pkgs.jdk21_headless}
             export JDK_DEV=${pkgs.graalvm-ce}
 
-            rm -f ./.env || true
+            rm ./.env/jdk || true
+            rmdir ./.env || true
             mkdir -p ./.env
-            ln -s ''${JDK_DEV} ./.env/jdk
+            ln -s ''${JDK_DEV} ./.env/jdk || true
           '';
         };
       }