test: fix Linux debug skip in SEA test guard

process.config.variables.is_debug is only populated by the GN build
flow (tools/generate_config_gypi.py), so on the gyp builds used by CI
the guard never fired and SEA tests ran on Linux debug builds against
the original intent. Switch to
process.config.target_defaults.default_configuration === 'Debug',
matching the pattern used in test/common/index.js.

Fixes: #63749
Refs: #61483
Signed-off-by: Hyunwoo Gu <realization0327@gmail.com>
PR-URL: #63751
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Richard Lau <richard.lau@ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: guhyunwoo

test/common/sea.js

@@ -20,7 +20,7 @@ function skipIfSingleExecutableIsNotSupported() {
   if (!['darwin', 'win32', 'linux'].includes(process.platform))
     common.skip(`Unsupported platform ${process.platform}.`);
 
-  if (process.platform === 'linux' && process.config.variables.is_debug === 1)
+  if (process.platform === 'linux' && common.isDebug)
     common.skip('Running the resultant binary fails with `Couldn\'t read target executable"`.');
 
   if (process.config.variables.node_shared)