[release/10.0] Defer DOTNET_DbgEnableMiniDump error message until dump creation (#126055)

Backport of #122986 to release/10.0

/cc @hoyosjs @Copilot

## Customer Impact

- [x] Customer reported
- [ ] Found internally

If customers specify the crash dump collecting variables on nativeAOT,
the default was to print error on startup to warn the user. It's also an
issue since we've moved tools like crossgen to nativeAOT, and setting
variables like this during build means child procs also prints the error
message - even if no crash is going to happen. This defers the message -
and while we are not proactive at letting the user know, it's a better
user experience for many cases where native AOT is used.

## Regression

- [ ] Yes
- [x] No

## Testing

Main has had this change for a while and is stable.

## Risk

Low. Changes eliminate unnecessary file system checks during
initialization and defer error detection to the point where the binary
is actually needed (execv call). The actual dump creation logic and all
success paths remain unchanged. Error messages now appear only when
dumps are actually attempted, preventing spurious warnings during
initialization.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: steveisok <471438+steveisok@users.noreply.github.com>
Co-authored-by: noahfalk <6243776+noahfalk@users.noreply.github.com>

Author: 4 people

src/coreclr/nativeaot/Runtime/unix/PalCreateDump.cpp

@@ -271,7 +271,14 @@ CreateCrashDump(
         // Execute the createdump program
         if (execv(argv[0], (char* const *)argv) == -1)
         {
-            fprintf(stderr, "Problem launching createdump (may not have execute permissions): execv(%s) FAILED %s (%d)\n", argv[0], strerror(errno), errno);
+            if (errno == ENOENT)
+            {
+                fprintf(stderr, "DOTNET_DbgEnableMiniDump is set and the createdump binary does not exist: %s\n", argv[0]);
+            }
+            else
+            {
+                fprintf(stderr, "Problem launching createdump (may not have execute permissions): execv(%s) FAILED %s (%d)\n", argv[0], strerror(errno), errno);
+            }
             exit(-1);
         }
     }
@@ -604,12 +611,6 @@ PalCreateDumpInitialize()
         }
         strncat(program, DumpGeneratorName, programLen);
 
-        struct stat fileData;
-        if (stat(program, &fileData) == -1 || !S_ISREG(fileData.st_mode))
-        {
-            fprintf(stderr, "DOTNET_DbgEnableMiniDump is set and the createdump binary does not exist: %s\n", program);
-            return true;
-        }
         g_szCreateDumpPath = program;
 
         // Format the app pid for the createdump command line