exec: check cdi manager not nil

[crazy-max]

fixes #5724

[tonistiigi]

    --- FAIL: TestExecOpCacheMap/empty (0.00s)
	github.com/moby/buildkit/solver/llbsolver/provenance		coverage: 0.0% of statements
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0xcfb8f2]

goroutine 19 [running]:
testing.tRunner.func1.2({0xdf7a60, 0x1711e60})
	/usr/local/go/src/testing/testing.go:1632 +0x230
testing.tRunner.func1()
	/usr/local/go/src/testing/testing.go:1635 +0x35e
panic({0xdf7a60?, 0x1711e60?})
	/usr/local/go/src/runtime/panic.go:785 +0x132
github.com/moby/buildkit/solver/llbsolver/ops.(*ExecOp).CacheMap(0xc00039e000, {0xc000383680?, 0x5af9cd?}, {0x15304f8?, 0x1111d2c?}, 0xf?)
	/src/solver/llbsolver/ops/exec.go:202 +0xa52
github.com/moby/buildkit/solver/llbsolver/ops.TestExecOpCacheMap.func1(0xc00039a1a0)
	/src/solver/llbsolver/ops/exec_test.go:114 +0xf3
testing.tRunner(0xc00039a1a0, 0xc00038e2c0)
	/usr/local/go/src/testing/testing.go:1690 +0xf4
created by testing.(*T).Run in goroutine 18
	/usr/local/go/src/testing/testing.go:1743 +0x390
?   	github.com/moby/buildkit/solver/llbsolver/provenance/types	[no test files]