adds platform env var related content from the spec

Signed-off-by: Hanan Younes <[email protected]>

content/docs/for-platform-operators/how-to/build-inputs/build-env.md

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+title="Specify the build time environment variables"
+weight=4
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+
+Environment variables are any values used to allow `Buildpacks` configurability. Some environment variables can't be modified while others are expected to get changed to allow a level of customization.
+
+<!--more-->
+
+## Buildpack Environment
+
+### Base Image-Provided Variables
+
+Environment variables that are set in the `lifecycle` execution environment and are inherited by the
+buildpack without any modification, such as:
+
+| Env Variable                | Description                    |
+|-----------------------------|--------------------------------|
+| `HOME`                      | Current user's home directory  |
+
+### POSIX Path Variables
+
+Environment variables that are set in the `lifecycle` execution environment. These variables can be modified by previous buildpacks before they being passed to the next buildpack:
+
+| Env Variable      | Layer Path   | Contents         |
+|-------------------|--------------|------------------|
+| `PATH`            | `/bin`       | binaries         |
+| `LD_LIBRARY_PATH` | `/lib`       | shared libraries |
+| `LIBRARY_PATH`    | `/lib`       | static libraries |
+| `CPATH`           | `/include`   | header files     |
+| `PKG_CONFIG_PATH` | `/pkgconfig` | pc files         |
+
+> Note: the `platform` does not assume any other `base-image` provided environment variables are inherited by the buildpack.
+
+### User-Provided Variables
+
+These variables are usually supplied by the `platform` as files in the `<platform>/env/` directory. Each file defines a single environment variable, where the file name defines the key and the file contents define the value.
+
+Only user-provided environment variables that are [POSIX path variables](#posix-path-variables) can be modified by prior buildpacks; however the user-provided value always get prepended to the buildpack-provided value.
+
+> Note: the `platform` does not set user-provided environment variables directly in the `lifecycle` execution environment.
+
+### Operator-Defined Variables
+
+The `platform` supplies Operator-defined environment variables as files in the `<build-config>/env/` directory.
+
+Operator-defined environment variables can be modified by previous buildpacks before getting passed to the following buildpack; however, the operator-defined value is always applied after the buildpack-provided value.
+
+> Note: the `platform` does not set operator-provided environment variables directly in the `lifecycle` execution environment.
+
+## Launch Environment
+
+User-provided modifications to the process execution environment are set directly in the `lifecycle` execution environment.
+
+The process inherits both `base-image-provided` and `user-provided` variables from the `lifecycle` execution environment with the following exceptions:
+
+* `CNB_APP_DIR`, `CNB_LAYERS_DIR` and `CNB_PROCESS_TYPE` SHALL NOT be set in the process execution environment.
+* `/cnb/process` SHALL be removed from the beginning of `PATH`.
+* The lifecycle SHALL apply buildpack-provided modifications to the environment as outlined in the [Buildpack Interface Specification](https://github.com/buildpacks/spec/blob/main/buildpack.md).