Port env var expension fix from rebar2

See https://github.com/rebar/rebar/pull/458

src/rebar_utils.erl

@@ -293,7 +293,7 @@ expand_env_variable(InStr, VarName, RawVarValue) ->
             VarValue = re:replace(RawVarValue, "\\\\", "\\\\\\\\", ReOpts),
             %% Use a regex to match/replace:
             %% Given variable "FOO": match $FOO\s | $FOOeol | ${FOO}
-            RegEx = io_lib:format("\\\$(~s(\\s|$)|{~s})", [VarName, VarName]),
+            RegEx = io_lib:format("\\\$(~s(\\W|$)|{~s})", [VarName, VarName]),
             re:replace(InStr, RegEx, [VarValue, "\\2"], ReOpts)
     end.