Fix relative_to="pkgsource" in generic build

[fantix]

include dir seems wrong in https://github.com/edgedb/edgedb/actions/runs/11944890842/job/33296624198:

2024-11-21T02:03:55.4341953Z   building 'edb.pgsql.parser.parser' extension
2024-11-21T02:03:55.4342598Z   creating build/temp.linux-x86_64-cpython-312/edb/pgsql
2024-11-21T02:03:55.4343137Z   creating build/temp.linux-x86_64-cpython-312/edb/pgsql/parser
2024-11-21T02:03:55.4352051Z   gcc -pthread -fno-strict-overflow -DNDEBUG -g -O3 -Wall -fno-semantic-interposition -g -O2 -D_FORTIFY_SOURCE=2 -fstack-protector-strong -Wdate-time -Wformat -Werror=format-security -I/tmp/metapkg.0to4_k89/edgedb-server-6-dev8997/edgedb-server/edgedb-server/../../../_artifacts/install/postgresql-edgedb/opt/edgedb-server-6-dev8997/include -I/tmp/metapkg.0to4_k89/edgedb-server-6-dev8997/edgedb-server/edgedb-server/../../../_artifacts/install/python-edgedb/opt/edgedb-server-6-dev8997/include -I/tmp/metapkg.0to4_k89/edgedb-server-6-dev8997/edgedb-server/edgedb-server/../../../_artifacts/install/pgext-pgvector/opt/edgedb-server-6-dev8997/include -I/tmp/metapkg.0to4_k89/edgedb-server-6-dev8997/edgedb-server/edgedb-server/../../../_artifacts/install/libprotobuf-c/opt/edgedb-server-6-dev8997/include -fPIC -I/tmp/metapkg.0to4_k89/edgedb-server-6-dev8997/edgedb-server/edgedb-server/edb/server/pgproto -I/tmp/metapkg.0to4_k89/edgedb-server-6-dev8997/edgedb-server/edgedb-server/edb/pgsql/parser/libpg_query -I/tmp/metapkg.0to4_k89/edgedb-server-6-dev8997/edgedb-server/thirdparty/python-edgedb/Include -I/tmp/metapkg.0to4_k89/edgedb-server-6-dev8997/_artifacts/build/python-edgedb -c edb/pgsql/parser/parser.c -o build/temp.linux-x86_64-cpython-312/edb/pgsql/parser/parser.o -O2 -Wno-error=incompatible-pointer-types -std=c99 -fsigned-char -Wall -Wsign-compare -Wconversion
2024-11-21T02:03:55.4792087Z   In file included from edb/pgsql/parser/parser.c:1280:
2024-11-21T02:03:55.4793457Z   /tmp/metapkg.0to4_k89/edgedb-server-6-dev8997/edgedb-server/edgedb-server/edb/pgsql/parser/libpg_query/protobuf/pg_query.pb-c.h:7:10: fatal error: protobuf-c/protobuf-c.h: No such file or directory
2024-11-21T02:03:55.4794562Z       7 | #include <protobuf-c/protobuf-c.h>
2024-11-21T02:03:55.4795128Z         |          ^~~~~~~~~~~~~~~~~~~~~~~~~
2024-11-21T02:03:55.4795436Z   compilation terminated.
2024-11-21T02:03:55.4803886Z   error: command '/opt/rh/devtoolset-11/root/usr/bin/gcc' failed with exit code 1
2024-11-21T02:03:55.5256802Z   error: subprocess-exited-with-error
2024-11-21T02:03:55.5257231Z   
2024-11-21T02:03:55.5261952Z   × Building wheel for gel-server (pyproject.toml) did not run successfully.
2024-11-21T02:03:55.5266576Z   │ exit code: 1
2024-11-21T02:03:55.5270237Z   ╰─> See above for output.

where the relevant include option resolves to:

-I/tmp/metapkg.0to4_k89/_artifacts/install/libprotobuf-c/opt/edgedb-server-6-dev8997/include

It's missing the edgedb-server-6-dev8997/ after /tmp/metapkg.0to4_k89/.

The problem is using the wrong package when relative_to="pkgsource" in the generic build get_build_install_dir() for root package dependencies.

Sample run