fix: escape individual items when constructing environment variables

[kmr-ankitt]

This pr implements the TODO to escape individual items before joining them into environment variables.

Before

items were joined directly:

env["IMPORT_PATHS"] = join(build_settings.importPaths, " ");

If a path contained spaces, e.g.

/home/user/my project/src

it could be interpreted as multiple shell arguments.

After

each item is escaped before joining, so paths and other values containing spaces are handled correctly.
this ensures paths and other values containing spaces are handled correctly.

[github-actions]

✅ PR OK, no changes in deprecations or warnings

Total deprecations: 0

Total warnings: 0

Build statistics:

 statistics (-before, +after)
 executable size=5710288 bin/dub
 rough build time=67s

Full build output

DUB version 1.41.0, built on Mar  1 2026
LDC - the LLVM D compiler (1.42.0):
  based on DMD v2.112.1 and LLVM 21.1.8
  built with LDC - the LLVM D compiler (1.42.0)
  Default target: x86_64-unknown-linux-gnu
  Host CPU: znver3
  https://dlang.org - https://wiki.dlang.org/LDC


  Registered Targets:
    aarch64     - AArch64 (little endian)
    aarch64_32  - AArch64 (little endian ILP32)
    aarch64_be  - AArch64 (big endian)
    amdgcn      - AMD GCN GPUs
    arm         - ARM
    arm64       - ARM64 (little endian)
    arm64_32    - ARM64 (little endian ILP32)
    armeb       - ARM (big endian)
    avr         - Atmel AVR Microcontroller
    bpf         - BPF (host endian)
    bpfeb       - BPF (big endian)
    bpfel       - BPF (little endian)
    hexagon     - Hexagon
    lanai       - Lanai
    loongarch32 - 32-bit LoongArch
    loongarch64 - 64-bit LoongArch
    mips        - MIPS (32-bit big endian)
    mips64      - MIPS (64-bit big endian)
    mips64el    - MIPS (64-bit little endian)
    mipsel      - MIPS (32-bit little endian)
    msp430      - MSP430 [experimental]
    nvptx       - NVIDIA PTX 32-bit
    nvptx64     - NVIDIA PTX 64-bit
    ppc32       - PowerPC 32
    ppc32le     - PowerPC 32 LE
    ppc64       - PowerPC 64
    ppc64le     - PowerPC 64 LE
    r600        - AMD GPUs HD2XXX-HD6XXX
    riscv32     - 32-bit RISC-V
    riscv64     - 64-bit RISC-V
    sparc       - Sparc
    sparcel     - Sparc LE
    sparcv9     - Sparc V9
    spirv       - SPIR-V Logical
    spirv32     - SPIR-V 32-bit
    spirv64     - SPIR-V 64-bit
    systemz     - SystemZ
    thumb       - Thumb
    thumbeb     - Thumb (big endian)
    ve          - VE
    wasm32      - WebAssembly 32-bit
    wasm64      - WebAssembly 64-bit
    x86         - 32-bit X86: Pentium-Pro and above
    x86-64      - 64-bit X86: EM64T and AMD64
    xcore       - XCore
    xtensa      - Xtensa 32
   Upgrading project in /home/runner/work/dub/dub/
    Starting Performing "release" build using /opt/hostedtoolcache/dc/ldc2-1.42.0/x64/ldc2-1.42.0-linux-x86_64/bin/ldc2 for x86_64.
    Building dub 1.41.0+commit.16.gb5c2f49f: building configuration [application]
     Linking dub
STAT:statistics (-before, +after)
STAT:executable size=5710288 bin/dub
STAT:rough build time=67s