bugfix: set correct workspace size parameter in build_engines.sh

[hebangwen]

The memPoolSize parameter in trtexec uses M instead of MiB.

Logs from trtexec with different memPoolSize:

memPoolSize	log	siglip latency
workspace:4096MiB	Memory Pools: workspace: 0.00390625 MiB	73.6ms
workspace:4096M	Memory Pools: workspace: 4096 MiB	72.1ms

---

Help message from trtexec --help:

  --memPoolSize=poolspec             Specify the size constraints of the designated memory pool(s)
                                     Supports the following base-2 suffixes: B (Bytes), G (Gibibytes), K (Kibibytes), M (Mebibytes).
                                     If none of suffixes is appended, the defualt unit is in MiB.
                                     Note: Also accepts decimal sizes, e.g. 0.25M. Will be rounded down to the nearest integer bytes.
                                     In particular, for dlaSRAM the bytes will be rounded down to the nearest power of 2.
                                   Pool constraint: poolspec ::= poolfmt[","poolspec]
                                                      poolfmt ::= pool:size
                                                    pool ::= "workspace"|"dlaSRAM"|"dlaLocalDRAM"|"dlaGlobalDRAM"|"tacticSharedMem"