nix: fix highlights

The `@` is now highlighted properly on either side of the function arg.

Also, extending the phases with `buildPhase = prev.buildPhase + ''''`
is now highlighted properly.

Fix highlighting of `''$` style escapes

runtime/queries/nix/highlights.scm

@@ -47,8 +47,10 @@
 (float_expression) @constant.numeric.float
 
 (escape_sequence) @constant.character.escape
+(dollar_escape) @constant.character.escape
 
 (function_expression
+  "@"? @punctuation.delimiter
   universal: (identifier) @variable.parameter
   "@"? @punctuation.delimiter
 )

runtime/queries/nix/injections.scm

@@ -5,9 +5,11 @@
 
 ((binding
    attrpath: (attrpath (identifier) @_path)
-   expression: (indented_string_expression
-     (string_fragment) @injection.content))
- (#match? @_path "(^\\w*Phase|(pre|post)\\w*|(.*\\.)?\\w*([sS]cript|[hH]ook)|(.*\\.)?startup)$")
+   expression: [
+     (indented_string_expression (string_fragment) @injection.content)
+     (binary_expression (indented_string_expression (string_fragment) @injection.content))
+   ])
+ (#match? @_path "(^\\w*Phase|command|(pre|post)\\w*|(.*\\.)?\\w*([sS]cript|[hH]ook)|(.*\\.)?startup)$")
  (#set! injection.language "bash")
  (#set! injection.combined))