There is an issue on Void Linux packages issue tracker about this bug[1]

# Steps to verify the issue on a fresh Git clone of QEMU source

$ git clone https://github.com/qemu/qemu
$ cd qemu
$ make DEBUG=1 docker-test-misc@alpine
[...]
  COPY    RUNNER
    RUN test-misc in qemu/alpine 
* Prepared to run command:
  ./test-misc
* Hit Ctrl-D to continue, or type 'exit 1' to abort

bash-5.1$ . common-rc
[...]
bash-5.1$ configure_qemu --disable-system --disable-user
Configure options:
--enable-werror --prefix=/tmp/qemu-test/install --disable-system --disable-user
cross containers  no
The Meson build system
Version: 0.56.2
Source dir: /tmp/qemu-test/src
Build dir: /tmp/qemu-test/src/tests/docker
Build type: native build
Project name: qemu
Project version: 5.2.50
[...]
bash-5.1$ grep filename pc-bios/descriptors/*
pc-bios/descriptors/50-edk2-i386-secure.json:            "filename": "share/qemu/edk2-i386-secure-code.fd",
pc-bios/descriptors/50-edk2-i386-secure.json:            "filename": "share/qemu/edk2-i386-vars.fd",
[...all other matches have relative paths...]
bash-5.1$ exit 1
[...]

# Research

The filename path substitution appears to be done in the file `pc-bios/descriptors/meson.build`.
In particular, occurrences of `@DATADIR@` in the template files get substituted using the value of `qemu_datadir`:

  configure_file(input: files(f),                                               
                 output: f,                                                     
                 configuration: {'DATADIR': qemu_datadir},                      
                 install: get_option('install_blobs'),                          
                 install_dir: qemu_datadir / 'firmware')

The variable `qemu_datadir` gets initialized from toplevel `meson.build` file using:

   qemu_datadir = get_option('datadir') / get_option('qemu_suffix')

From the Meson documentation on built-in (build) options[2], `datadir` option gets initialized to `"share"` (note: a relative path!) by default, unless `datadir` build option is configured explicitly, so the value of `qemu_datadir`, as well as the substitution made ends up being a relative path as well.

I found a commit which I think is relevant to why this choice was made[3].

# Test a workaround, try #1: specify `datadir` manually

$ make DEBUG=1 docker-test-misc@alpine
[...]
bash-5.1$ configure_qemu --datadir=/usr/share --disable-system --disable-user
Configure options:
--enable-werror --prefix=/tmp/qemu-test/install --datadir=/usr/share --disable-system --disable-user
cross containers  no
The Meson build system
Version: 0.56.2
Source dir: /tmp/qemu-test/src
Build dir: /tmp/qemu-test/src/tests/docker
Build type: native build

../../meson.build:1:0: ERROR: The value of the 'datadir' option is '/usr/share' which must be a subdir of the prefix '/tmp/qemu-test/install'.
Note that if you pass a relative path, it is assumed to be a subdir of prefix.

A full log can be found at /tmp/qemu-test/src/tests/docker/meson-logs/meson-log.txt

ERROR: meson setup failed

Ah! So perhaps `datadir` can be an absolute path, but then it must be a subdir of the (already specified) `prefix`.
Let's try again.

# Test a workaround, try #2: specify `datadir` as subdir of <prefix> manually

$ make DEBUG=1 docker-test-misc@alpine
[...]
bash-5.1$ configure_qemu --datadir=/tmp/qemu-test/install/share --disable-system --disable-user
Configure options:
--enable-werror --prefix=/tmp/qemu-test/install --datadir=/tmp/qemu-test/install/share --disable-system --disable-user
[...]
bash-5.1$ grep filename pc-bios/descriptors/*
pc-bios/descriptors/50-edk2-i386-secure.json:            "filename": "share/qemu/edk2-i386-secure-code.fd",
pc-bios/descriptors/50-edk2-i386-secure.json:            "filename": "share/qemu/edk2-i386-vars.fd",
bash-5.1$ exit 1

Getting there, but the paths are still relative, missing the initial `/`.

[1]: https://github.com/void-linux/void-packages/issues/27965
[2]: https://mesonbuild.com/Builtin-options.html
[3]: https://github.com/qemu/qemu/commit/ab4c0996f80d43d1fc28c6e76f4ecb27423a7e29