Fix syntax for RDMA_CNI_IMAGE var substitution

[mandre]

The bash syntax was incorrect and yielded:

hack/env.sh: line 35: ${$RDMA_CNI_IMAGE:-}: bad substitution

[github-actions]

Thanks for your PR,
To run vendors CIs, Maintainers can use one of:

• /test-all: To run all tests for all vendors.
• /test-e2e-all: To run all E2E tests for all vendors.
• /test-e2e-nvidia-all: To run all E2E tests for NVIDIA vendor.

To skip the vendors CIs, Maintainers can use one of:

• /skip-all: To skip all tests for all vendors.
• /skip-e2e-all: To skip all E2E tests for all vendors.
• /skip-e2e-nvidia-all: To skip all E2E tests for NVIDIA vendor.
  Best regards.

[coveralls]

Pull Request Test Coverage Report for Build 10970625017

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.01%) to 45.048%

---

Files with Coverage Reduction	New Missed Lines	%
controllers/drain_controller.go	1	68.06%

Totals
Change from base Build 10955688294:	0.01%
Covered Lines:	6627
Relevant Lines:	14711

---

💛 - Coveralls