Merge bitcoin/bitcoin#24279: build: Make `$(package)_*_env` available to all `$(package)_*_cmds`

affbf58a1e52a8e60c830be6a9e0347e0ff4c28e build: Move environment variables into `$(package)_config_env` (Hennadii Stepanov)
d44fcd3c976572883bbf7f386bc88e2610dc1a58 build: Make $(package)_*_env available to all $(package)_*_cmds (Hennadii Stepanov)

Pull request description:

  On master (1e7564eca8a688f39c75540877ec3bdfdde766b1) the depends build system, which is based on pure GNU Make, works, but it lacks robustness, and in some corner cases it fails. For example, see bitcoin/bitcoin#22552.

  Another [bug](https://github.com/bitcoin/bitcoin/issues/22719) in the depends build system has already become a problem at least two times in the past (https://github.com/bitcoin/bitcoin/pull/16883#issuecomment-683817472 and https://github.com/bitcoin/bitcoin/pull/24134). Each time the problem was solved with other means.

  The initial [solution](https://github.com/bitcoin/bitcoin/pull/19882) had some discussion. Also it was discussed on the IRC meeting in #bitcoin-core-builds channel. This PR, actually, is a resurrection of it, as the bug silently struck pretty [recently](https://github.com/bitcoin/bitcoin/pull/24134).

  The bug is well described in bitcoin/bitcoin#22719.

  Here is another, a bit simpler description, which requires only basic shell (bash, dash etc) experience.
  After creating targets by this code:https://github.com/bitcoin/bitcoin/blob/1e7564eca8a688f39c75540877ec3bdfdde766b1/depends/funcs.mk#L280 a "draft" line of recipe like `$($(1)_config_env) $(call $(1)_config_cmds, $(1))` becomes a shell command sequence `VAR1=foo VAR2=bar command1 && command2` which is supposed to be executed in a [new sub-shell](https://www.gnu.org/software/make/manual/html_node/Execution.html#Execution).

  Please note that `VAR1=foo VAR2=bar` part is visible for the first `command1` only (for details see shell docs). Example:
  ```sh
  $ VAR1="foo" VAR2="bar" echo "begin" && printenv VAR1 && printenv VAR2 && echo "end"
  begin
  $ echo $?
  1
  ```

  Using the `export` command is a trivial solution:
  ```sh
  $ export VAR1="foo" VAR2="bar"; echo "begin" && printenv VAR1 && printenv VAR2 && echo "end"
  begin
  foo
  bar
  end
  $ echo $?
  0
  ```

  As a [new sub-shell](https://www.gnu.org/software/make/manual/html_node/Execution.html#Execution) is invoked for each line of the recipe, there are no side effects of using `export`. It means this solution should not be considered invasive.

  Fixes bitcoin/bitcoin#22719.

  ---

  Also this PR removes no longer needed crutch from `qt.mk`.

ACKs for top commit:
  fanquake:
    ACK affbf58a1e52a8e60c830be6a9e0347e0ff4c28e

Tree-SHA512: 0ce2cf82870a7774bdf1592fac50857126ae47da902e349f1092d50109223be9d6a8efd5e92ec08c2ca775b17516482aabaf232378950ade36484a883acc177b

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