[ISSUE-17382] Fix for "podman build --secrets with environment variables does not work on MacOS"

[apcro]

Fixes: #17382

A fix for [Issue 17382](https://github.com/containers/podman/issues/17382) 

This PR specifically treats a passed `env=` directive to `--secrets` as if it 
were passed inside a `src` file, and returns the same content when referenced 
during the container `build` process.

Testing using a shell

Create a Dockerfile with the following content

FROM python:3.9-slim-bullseye
RUN --mount=type=secret,id=mysecret echo "Secret is" && cat /run/secrets/mysecret

Create a secrets.txt file with the following content

filebasedsecret

Add an environment variable
TEST_VAR=envvarsecret

Once you have these files, run the following in your terminal:

Using an ENV var
path/to/podman build --secret id=mysecret,env=TEST_VAR --no-cache .

Expected output

STEP 1/2: FROM python:3.9-slim-bullseye
STEP 2/2: RUN --mount=type=secret,id=mysecret echo "Secret is" && cat /run/secrets/mysecret
Secret is
envvarsecret

Using the secrets file
path/to/podman build --secret id=mysecret,src=secrets.txt --no-cache .

Expected output:

STEP 1/2: FROM python:3.9-slim-bullseye
STEP 2/2: RUN --mount=type=secret,id=mysecret echo "Secret is" && cat /run/secrets/mysecret
Secret is
filebasedsecret

[openshift-ci]

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[packit-as-a-service]

Ephemeral COPR build failed. @containers/packit-build please check.

[apcro]

I don't believe there are any new tests needed, however I can't check as the provided testing tools are failing.

> make localintegration FOCUS_FILE=secret_test.go
Makefile:152: invalid `override' directive
[...]
# github.com/containers/podman/v5/test/checkseccomp
test/checkseccomp/checkseccomp.go:13:17: undefined: unix.Prctl
test/checkseccomp/checkseccomp.go:13:28: undefined: unix.PR_GET_SECCOMP
test/checkseccomp/checkseccomp.go:15:18: undefined: unix.Prctl
test/checkseccomp/checkseccomp.go:15:29: undefined: unix.PR_SET_SECCOMP
test/checkseccomp/checkseccomp.go:15:50: undefined: unix.SECCOMP_MODE_FILTER
make: *** [test/checkseccomp/checkseccomp] Error 1