Only update index map when needed #6286

Job	Run time
Check tket library version	14s
Build and test pytket (macos) (macos-13)	11m 2s
Build and test pytket (ubuntu)	10m 39s
Build and test pytket (windows)	14m 35s
Build and test pytket (macos) (macos-15)	7m 57s
Build and test (tket) (ubuntu-24.04)	1m 50s
Build and test (tket) (macos-13)	3m 2s
Build and test (tket) (windows-2022)	36m 23s
Check C++ code formatting	24s
Build and test (tket) (macos-15)	1m 22s
Check documentation build	53s
Check pytket line and branch coverage	4s
Publish pytket coverage	0s
All build and test jobs successful (or skipped)	1s
	1h 28m 26s

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