Introduce os.proc.env DynamicVariable to override default env

In Mill and any client-server application, it is handy to start processes on the server passing the
client environment. This requires to pass the environment manually in all the `os.proc(...).call()`s.
An easier alternative is to override the environment as we to for `os.Inherit` `out`, `in` and `err` pipes.
So users can forget about the environment and have the right one passed automatically.

os/src/ProcessOps.scala

@@ -189,6 +189,10 @@ case class proc(command: Shellable*) {
   def pipeTo(next: proc): ProcGroup = ProcGroup(Seq(this, next))
 }
 
+object proc {
+  val env = new scala.util.DynamicVariable[Map[String, String]](sys.env)
+}
+
 /**
  * A group of processes that are piped together, corresponding to e.g. `ls -l | grep .scala`.
  * You can create a `ProcGroup` by calling `.pipeTo` on a [[proc]] multiple times.
@@ -423,18 +427,22 @@ private[os] object ProcessOps {
     val builder = new java.lang.ProcessBuilder()
 
     val environment = builder.environment()
+    environment.clear()
 
-    if (!propagateEnv) {
-      environment.clear()
-    }
-
-    if (env != null) {
-      for ((k, v) <- env) {
-        if (v != null) builder.environment().put(k, v)
-        else builder.environment().remove(k)
+    def addToProcessEnv(env: Map[String, String]) =
+      if (env != null) {
+        for ((k, v) <- env) {
+          if (v != null) builder.environment().put(k, v)
+          else builder.environment().remove(k)
+        }
       }
+
+    if (propagateEnv) {
+      addToProcessEnv(os.proc.env.value)
     }
 
+    addToProcessEnv(env)
+
     builder.directory(Option(cwd).getOrElse(os.pwd).toIO)
 
     builder

os/test/src/SubprocessTests.scala

@@ -146,6 +146,22 @@ object SubprocessTests extends TestSuite {
         }
       }
     }
+    test("envWithValue") {
+      if (Unix()) {
+        def envValue() = os.proc("bash", "-c", "echo \"$TEST_ENV_FOO\"").call().out.lines().head
+
+        val before = envValue()
+        assert(before == "")
+
+        os.proc.env.withValue(Map("TEST_ENV_FOO" -> "bar")) {
+          val res = envValue()
+          assert(res == "bar")
+        }
+
+        val after = envValue()
+        assert(after == "")
+      }
+    }
     test("multiChunk") {
       // Make sure that in the case where multiple chunks are being read from
       // the subprocess in quick succession, we ensure that the output handler