cmd/ssadump: disable run mode with runtime package
cmd/ssadump: disable run mode with runtime package

Changes `ssadump -run` to ensure that the package runtime
is not imported (changed from must be imported). For several
years, the runtime package has used unsafe constructs
x/tools/go/ssa/interp cannot interpret. This must have been
failing a similar amount of time.

This is unfortunate, but is unlikely to be addressed soon.

For golang/go#43163

Change-Id: I9e2aee640ff7b1123e591e6c49cac9967c5e8da8
Reviewed-on: https://go-review.googlesource.com/c/tools/+/441817
Run-TryBot: Tim King <ee8a1d1efb05d8171fc0140ced15d455e83941c5@google.com>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Alan Donovan <7069e9552f9d110b0a0f326b96f22f63fb99dfc1@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools