zig env: remove the json output

The current json output is not very convenient to use from the shell or
a Zig program.
An example using the shell: `zig env | jq -r .lib_dir`.

Remove the json output, and instead use the POSIX shell syntax:
  name="value"

Additionally, when args is not empty, assume each argument is the
environment variable name and print the associated value.
Unrecognized environment variables are ignored.

The new output format has been copied from `go env`, with the difference
that `go env` uses OS specific syntax for windows and plan9.

Define the environment variables in a single place, in order to avoid
possible bugs.

src/print_env.zig

@@ -1,11 +1,16 @@
 const std = @import("std");
+const mem = std.mem;
 const build_options = @import("build_options");
 const introspect = @import("introspect.zig");
 const Allocator = std.mem.Allocator;
 const fatal = @import("main.zig").fatal;
 
+const Env = struct {
+    name: []const u8,
+    value: []const u8,
+};
+
 pub fn cmdEnv(gpa: Allocator, args: []const []const u8, stdout: std.fs.File.Writer) !void {
-    _ = args;
     const self_exe_path = try introspect.findZigExePath(gpa);
     defer gpa.free(self_exe_path);
 
@@ -25,32 +30,33 @@ pub fn cmdEnv(gpa: Allocator, args: []const []const u8, stdout: std.fs.File.Writ
     const triple = try info.target.zigTriple(gpa);
     defer gpa.free(triple);
 
+    const envars: []Env = &[_]Env{
+        .{ .name = "zig_exe", .value = self_exe_path },
+        .{ .name = "lib_dir", .value = zig_lib_directory.path.? },
+        .{ .name = "std_dir", .value = zig_std_dir },
+        .{ .name = "global_cache_dir", .value = global_cache_dir },
+        .{ .name = "version", .value = build_options.version },
+        .{ .name = "target", .value = triple },
+    };
+
     var bw = std.io.bufferedWriter(stdout);
     const w = bw.writer();
 
-    var jws = std.json.writeStream(w, .{ .whitespace = .indent_1 });
+    if (args.len > 0) {
+        for (args) |name| {
+            for (envars) |env| {
+                if (mem.eql(u8, name, env.name)) {
+                    try w.print("{s}\n", .{env.value});
+                }
+            }
+        }
+        try bw.flush();
 
-    try jws.beginObject();
+        return;
+    }
 
-    try jws.objectField("zig_exe");
-    try jws.write(self_exe_path);
-
-    try jws.objectField("lib_dir");
-    try jws.write(zig_lib_directory.path.?);
-
-    try jws.objectField("std_dir");
-    try jws.write(zig_std_dir);
-
-    try jws.objectField("global_cache_dir");
-    try jws.write(global_cache_dir);
-
-    try jws.objectField("version");
-    try jws.write(build_options.version);
-
-    try jws.objectField("target");
-    try jws.write(triple);
-
-    try jws.endObject();
-    try w.writeByte('\n');
+    for (envars) |env| {
+        try w.print("{[name]s}=\"{[value]s}\"\n", env);
+    }
     try bw.flush();
 }