fix(ci): align MinIO secret in env files with Go-side widened key

Matches commit 220a9ce5d on the fork. The Go code at:
  systest/bulk_live/common/bulk_live_fixture.go:128
  systest/backup/encryption/backup_test.go:112
  systest/backup/minio/backup_test.go:108
  systest/cloud/cloud_test.go:165,169
  testutil/graphql.go:284
  testutil/minio.go

widens MINIO_SECRET_KEY from "secretkey" to "secretkey-long-enough"
(part of the HMAC-widening commit 8d185f9). But the MinIO server
configured by the corresponding .env files still used "secretkey",
causing TestBulkCases, TestBackupMinioE, TestBackupMinio to fail with
"The request signature we calculated does not match the signature you
provided" when the Go client tries to sign with the widened key
against a MinIO server expecting the narrow one.

Fix: update MINIO_SECRET_KEY to "secretkey-long-enough" in all three
.env files:
  - dgraph/minio.env
  - systest/backup.env
  - systest/export/export.env

Server and client now agree. Observed failing in upstream PR CI
runs 24878931876 (dgraph-load-tests) and 24878931803 (dgraph-systest-tests).

Author: mlwelles

dgraph/minio.env

@@ -1,2 +1,2 @@
 MINIO_ACCESS_KEY=accesskey
-MINIO_SECRET_KEY=secretkey
+MINIO_SECRET_KEY=secretkey-long-enough

systest/backup.env

@@ -1,2 +1,2 @@
 MINIO_ACCESS_KEY=accesskey
-MINIO_SECRET_KEY=secretkey
+MINIO_SECRET_KEY=secretkey-long-enough

systest/export/export.env

@@ -1,2 +1,2 @@
 MINIO_ACCESS_KEY=accesskey
-MINIO_SECRET_KEY=secretkey
+MINIO_SECRET_KEY=secretkey-long-enough