make can raise bin/sh: line 1: 1.2.md: No such file or directory

[m-schmoock]

Issue and Steps to Reproduce

At least on some systems, calling make can raise an shell error bin/sh: line 1: 1.2.md: No such file or directory
This seems to be caused by doc/Makefile line 215 which has been brought in by 744d111 in early December 2022.

doc/index.rst: $(MANPAGES:=.md)
	@$(call VERBOSE, "genidx $@", \
	  find doc -maxdepth 1 -name '*\.[0-9]\.md' | \
	  cut -b 5- | LC_ALL=C sort | \
	  sed 's/\(.*\)\.\(.*\).*\.md/\1 <\1.\2.md>/' | \
	  python3 devtools/blockreplace.py doc/index.rst manpages --language=rst --indent "   " \
	)

make output

LD: cc     config.vars  -Lexternal/x86_64-pc-linux-gnu -lwallycore -lsecp256k1 -ljsmn -lbacktrace -lsodium -L/usr/local/include -lm -lgmp -lsqlite3  -lz  -L/usr/lib -lpq -o
/bin/sh: line 1: 1.2.md: No such file or directory

Sometimes (maybe because of parallel compiling) this error is not at the end of the output but a couple of lines before.