Remove nondeterminism in pck_packer

PCK files (like other build products) should be deterministic based on their inputs. Removed calls to Math::rand() that are being used to generate padding.

Looks like these were introduced as part of adding encryption support, but the padding being random does not have any cryptographic significance. This can be trivially inferred since file blocks that happen to be aligned don't get padding anyway.

If there's a desire to indroduce something that functions as a nonce it should probably be added explicitly and only if encryption is enabled.
remove Math::rand() calls in editor_export_platform.cpp

follow up to make consistent with pck_packer

(cherry picked from commit 067807c1cb7a13624c1d226f301b0431c6edec87)

core/io/pck_packer.cpp

@@ -205,7 +205,7 @@ Error PCKPacker::flush(bool p_verbose) {
 
 	int header_padding = _get_pad(alignment, file->get_position());
 	for (int i = 0; i < header_padding; i++) {
-		file->store_8(Math::rand() % 256);
+		file->store_8(0);
 	}
 
 	int64_t file_base = file->get_position();
@@ -244,7 +244,7 @@ Error PCKPacker::flush(bool p_verbose) {
 
 		int pad = _get_pad(alignment, file->get_position());
 		for (int j = 0; j < pad; j++) {
-			file->store_8(Math::rand() % 256);
+			file->store_8(0);
 		}
 
 		count += 1;

editor/export/editor_export_platform.cpp

@@ -233,7 +233,7 @@ Error EditorExportPlatform::_save_pack_file(void *p_userdata, const String &p_pa
 
 	int pad = _get_pad(PCK_PADDING, pd->f->get_position());
 	for (int i = 0; i < pad; i++) {
-		pd->f->store_8(Math::rand() % 256);
+		pd->f->store_8(0);
 	}
 
 	// Store MD5 of original file.
@@ -1656,7 +1656,7 @@ Error EditorExportPlatform::save_pack(const Ref<EditorExportPreset> &p_preset, b
 
 	int header_padding = _get_pad(PCK_PADDING, f->get_position());
 	for (int i = 0; i < header_padding; i++) {
-		f->store_8(Math::rand() % 256);
+		f->store_8(0);
 	}
 
 	uint64_t file_base = f->get_position();